fix some bugs from the user groups PR

[prandla]

fixes some places that still tried to use contest.stop instead of group.stop. also removes the per_user_time field that was for some reason still left in the Contest class.

@pxsit ran into these in his PR.

[prandla]

(very funny how the bug in check.py wasn't caught by unit tests, because the unit tests were mocking the old interface instead of the new one.......)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.68%. Comparing base (efaab69) to head (865db34).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1672      +/-   ##
==========================================
- Coverage   54.69%   54.68%   -0.01%     
==========================================
  Files         336      336              
  Lines       27440    27439       -1     
==========================================
- Hits        15007    15005       -2     
- Misses      12433    12434       +1     

Flag	Coverage Δ
functionaltests	0.00% <0.00%> (ø)
unittests	54.68% <100.00%> (-0.01%)	⬇️

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