Fix #4347: Add is_cli flag to track CLI vs UI submissions

[vishwa21-prog]

Changes:

Added is_cli field to Submission model
Added is_cli to SubmissionSerializer and ChallengeSubmissionManagementSerializer
Created migration file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.33%. Comparing base (b438c28) to head (8a79ffc).
⚠️ Report is 19 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5038   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files          91       91           
  Lines        7671     7671           
=======================================
  Hits         7083     7083           
  Misses        588      588           

Flag	Coverage Δ
backend	96.76% <ø> (ø)
frontend	87.46% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Accounts & Authentication	97.79% <ø> (ø)
Challenges Management	95.51% <ø> (ø)
Job Processing	98.85% <ø> (ø)
Participants & Teams	100.00% <ø> (ø)
Challenge Hosts	100.00% <ø> (ø)
Analytics	100.00% <ø> (ø)
Web Interface	100.00% <ø> (ø)
Frontend (Gulp)	87.46% <ø> (ø)
All Models	97.73% <ø> (ø)
All Views	100.00% <ø> (ø)
All Serializers	98.57% <ø> (ø)
Utility Functions	97.14% <ø> (ø)
Core Configuration	82.35% <ø> (ø)

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