[Multiple choice questions] Make "other" text required

[shobhitagarwal1612]

Fixes #2833

Disables the previous/next buttons if the task is required and user has selected other option but hasn't filled the text.

Verified by running the app locally and via unit tests

@gino-m PTAL?

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 61.19%. Comparing base (ca9e4bd) to head (a090df9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...asks/multiplechoice/MultipleChoiceTaskViewModel.kt	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #2870   +/-   ##
=========================================
  Coverage     61.19%   61.19%           
- Complexity     1177     1182    +5     
=========================================
  Files           266      266           
  Lines          6345     6345           
  Branches        906      904    -2     
=========================================
  Hits           3883     3883           
  Misses         1933     1933           
  Partials        529      529           

Files with missing lines	Coverage Δ
...asks/multiplechoice/MultipleChoiceTaskViewModel.kt	92.64% <88.88%> (ø)

[shobhitagarwal1612]

@gino-m PTAL

[gino-m]

Much clearer, thank you! 🙏