Use django.utils.timezone.now instead of datetime.now for timez…

[jezdez]

We should make sure to always use django.utils.timezone.now instead of datetime.datetime.now so datetimes for database content are timezone-aware. datetimes created by datetime.datetime.now don't have any tzinfo and are basically creating race conditions.

Pull Request checklist

• Documentation: If this feature has or requires documentation, the relevant docs have been updated.
• Changelog: This PR updates the changelog with any user-visible changes.
• Tests: This PR includes thorough tests or an explanation of why it does not

[wlach]

LGTM though the unit tests are failing. Also same comment about the redundant unreleased header in the changelog that I noted last time :)

[codecov-io]

Codecov Report

Merging #2323 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2323      +/-   ##
==========================================
+ Coverage   69.34%   69.35%   +0.01%     
==========================================
  Files         220      220              
  Lines        5333     5335       +2     
  Branches      854      854              
==========================================
+ Hits         3698     3700       +2     
  Misses       1606     1606              
  Partials       29       29

Impacted Files	Coverage Δ
server/files/tasks.py	100% <100%> (ø)	⬆️
server/tests/test_file_update_operations.py	100% <100%> (ø)	⬆️

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