Run submission after end date

[poju3185]

This PR adds a new boolean field, submission_must_finish_before_end, to the Challenge model, allowing challenges to accept submissions that are uploaded before the challenge ends, even if processing happens later. For operational efficiency, a constraint has been introduced to only apply this leniency to challenges ending within the last 3 days, though this duration is open for adjustment based on feedback.

[codecov-commenter]

Codecov Report

Merging #4306 (e7fe4f5) into master (96968d6) will decrease coverage by 3.63%.
Report is 1101 commits behind head on master.
The diff coverage is n/a.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4306      +/-   ##
==========================================
- 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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