Add a regular, non-unique index on lower(gh_login)

Author: carols10cents

migrations/2017-10-09-135625_add_lower_gh_login_index/down.sql

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+DROP INDEX IF EXISTS lower_gh_login;
+CREATE INDEX index_users_gh_login ON users (gh_login);

migrations/2017-10-09-135625_add_lower_gh_login_index/up.sql

@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS index_users_gh_login;
+CREATE INDEX lower_gh_login ON users (lower(gh_login));