Flatten authors and contributors array

The authors and contributors of exercises are stored in an array in the exercises' `.meta/config.json` files.
We used to have separate fields for the GitHub- and Exercism username of the user.
As we're only using the GitHub username, we've flatten the `authors` and `contributors` array to an array of strings representing the GitHub usernames.

See https://github.com/exercism/docs/pull/90/files

Author: ErikSchierboom

exercises/concept/basketball-website/.meta/config.json

@@ -1,20 +1,11 @@
 {
   "blurb": "Learn about Access Behaviour by helping extract deeply nested data for the basketball team's website.",
   "authors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "contributors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    },
-    {
-      "github_username": "NobbZ",
-      "exercism_username": "NobbZ"
-    }
+    "angelikatyborska",
+    "NobbZ"
   ],
   "files": {
     "solution": [

exercises/concept/bird-count/.meta/config.json

@@ -1,16 +1,10 @@
 {
   "blurb": "Learn about recursion by keeping track of how many birds visit your garden each day.",
   "authors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    }
+    "angelikatyborska"
   ],
   "contributors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "files": {
     "solution": [

exercises/concept/boutique-inventory/.meta/config.json

@@ -1,16 +1,10 @@
 {
   "blurb": "Learn about the Enum module by preparing your fashion boutique for the big annual sale.",
   "authors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    }
+    "angelikatyborska"
   ],
   "contributors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "files": {
     "solution": [

exercises/concept/boutique-suggestions/.meta/config.json

@@ -1,16 +1,10 @@
 {
   "blurb": "Learn about list comprehensions by generating outfit suggestions for the clients of your fashion boutique.",
   "authors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "contributors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    }
+    "angelikatyborska"
   ],
   "files": {
     "solution": [

exercises/concept/bread-and-potions/.meta/config.json

@@ -1,16 +1,10 @@
 {
   "blurb": "Learn about protocols by developing your own role-playing video game.",
   "authors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    }
+    "angelikatyborska"
   ],
   "contributors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "files": {
     "solution": [

exercises/concept/captains-log/.meta/config.json

@@ -1,16 +1,10 @@
 {
   "blurb": "Learn about randomness and using Erlang libraries from Elixir by helping Mary generate stardates and starship registry numbers for her Star Trek themed pen-and-paper role playing sessions.",
   "authors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    }
+    "angelikatyborska"
   ],
   "contributors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "files": {
     "solution": [

exercises/concept/chessboard/.meta/config.json

@@ -1,16 +1,10 @@
 {
   "blurb": "Learn about ranges by generating a chessboard.",
   "authors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    }
+    "angelikatyborska"
   ],
   "contributors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "files": {
     "solution": [

exercises/concept/community-garden/.meta/config.json

@@ -1,24 +1,12 @@
 {
   "blurb": "Learn about the Agent module by helping your local community handle community garden registrations.",
   "authors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "contributors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    },
-    {
-      "github_username": "jrr",
-      "exercism_username": "jrr"
-    },
-    {
-      "github_username": "efx",
-      "exercism_username": "efx"
-    }
+    "angelikatyborska",
+    "jrr",
+    "efx"
   ],
   "files": {
     "solution": [

exercises/concept/date-parser/.meta/config.json

@@ -1,16 +1,10 @@
 {
   "blurb": "Learn about regular expressions by parsing dates.",
   "authors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "contributors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    }
+    "angelikatyborska"
   ],
   "files": {
     "solution": [

exercises/concept/dna-encoding/.meta/config.json

@@ -1,20 +1,11 @@
 {
   "blurb": "Learn about bitstrings and tail call recursion by encoding DNA sequences as binary data.",
   "authors": [
-    {
-      "github_username": "neenjaw",
-      "exercism_username": "neenjaw"
-    }
+    "neenjaw"
   ],
   "contributors": [
-    {
-      "github_username": "angelikatyborska",
-      "exercism_username": "angelikatyborska"
-    },
-    {
-      "github_username": "NobbZ",
-      "exercism_username": "NobbZ"
-    }
+    "angelikatyborska",
+    "NobbZ"
   ],
   "files": {
     "solution": [