API for challenge tags

[pushkalkatara]

Fixes #2735

• Add a tags field (postgres array field, default=[], null=True) to Challenges Model

• List tags associated with a challenge on the challenge list page, challenge details page

• Add tags support in Challenge creation templates so that tags automatically gets created when configuration is uploaded to EvalAI

• Update/add unit tests for tags

Output:

[pushkalkatara]

corresponding PR for frontend - Cloud-CV/EvalAI-ngx#313

[pushkalkatara]

corresponding PR for challenge config - Cloud-CV/EvalAI-Starters#19

[pushkalkatara]

Hi @RishabhJain2018 @deshraj

Please review the PR.

[RishabhJain2018]

Closing this since we already have this feature now!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.66%. Comparing base (96968d6) to head (8503b42).
Report is 1120 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2739      +/-   ##
==========================================
- Coverage   72.93%   72.66%   -0.27%     
==========================================
  Files          83       84       +1     
  Lines        5368     5425      +57     
==========================================
+ Hits         3915     3942      +27     
- Misses       1453     1483      +30     

Files with missing lines	Coverage Δ
apps/challenges/models.py	99.48% <100.00%> (+0.02%)	⬆️
apps/challenges/serializers.py	95.96% <ø> (+0.13%)	⬆️
apps/challenges/urls.py	100.00% <ø> (ø)
apps/challenges/views.py	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

Files with missing lines	Coverage Δ
apps/challenges/models.py	99.48% <100.00%> (+0.02%)	⬆️
apps/challenges/serializers.py	95.96% <ø> (+0.13%)	⬆️
apps/challenges/urls.py	100.00% <ø> (ø)
apps/challenges/views.py	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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