Add challenge approval from Slack message

[MinhThieu145]

This pull request adds the functionality to approve or disapprove challenges from Slack messages. It includes changes to the Django views and URLs to handle the approval actions and updates the Slack notification message to include buttons for approval and disapproval.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.30%. Comparing base (96968d6) to head (2cae300). Report is 1110 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4388      +/-   ##
==========================================
- Coverage   72.93%   69.30%   -3.63%     
==========================================
  Files          83       20      -63     
  Lines        5368     3574    -1794     
==========================================
- Hits         3915     2477    -1438     
+ Misses       1453     1097     -356     

see 64 files with indirect coverage changes

see 64 files with indirect coverage changes

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