closes #438, #439 - return name in getLessonMentors resolver, create validateLessonId function

[anthonykhoa]

Issues

#438
In our designs, the list of lesson mentors has both the mentor's username and name. Currently the getLessonMentors resolver only returns a mentor's username, and so it needs to be updated to also return the mentor's name.

#439
The lessonExists function is only being used to see if an error message should be thrown or not. Since this logic of check if lesson exists -> throw error is repetitive in the codebase, a function should handle this logic.

This PR will:

• Update getLessonMentors to also return a mentor's name
• Delete lessonExists and create validateLessonId function
• Refactor where necessary in codebase to use validateLessonId instead of lessonExists function

[vercel]

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[codecov]

Codecov Report

Merging #440 (788edbe) into master (d677cdc) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #440   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           86        86           
  Lines         1137      1139    +2     
  Branches       251       250    -1     
=========================================
+ Hits          1137      1139    +2     

Impacted Files	Coverage Δ
graphql/queryResolvers/getLessonMentors.ts	100.00% <100.00%> (ø)
helpers/controllers/challengesController.ts	100.00% <100.00%> (ø)
helpers/controllers/lessonsController.ts	100.00% <100.00%> (ø)
helpers/validateLessonId.ts	100.00% <100.00%> (ø)