addExerciseComment mutation

[HS-90]

Description: relates to #2400

This PR adds a GraphQL mutation and resolver for adding Exercise comments(Discussion feature). To test:

1. Go to 'api/graphql'
2. write a query as follows:
   mutation AddExerciseComment{ addExerciseComment(exerciseId:1, content:"Cool"){ id exerciseId content } }
3. You should receive a response as such:
   { "data": { "addExerciseComment": { "id": 3, "exerciseId": 1, "content": "Cool" } } }

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Name	Status	Preview	Updated
c0d3-app	✅ Ready (Inspect)	Visit Preview	Oct 29, 2022 at 2:25AM (UTC)

[codecov]

Codecov Report

Merging #2468 (1ba50f2) into master (2afa052) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2468   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          185       186    +1     
  Lines         3311      3316    +5     
  Branches       879       880    +1     
=========================================
+ Hits          3311      3316    +5     

Impacted Files	Coverage Δ
graphql/resolvers/exerciseCommentCrud.ts	100.00% <100.00%> (ø)

[flacial]

Looks good to me!