Consolidate notification duplicates

[nijel]

Proposed changes

• Make the code reusable
• Filter out duplicate translation notifications (fixes Duplicate notifications #9464)

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

Merging #9483 (58d7de1) into main (69f5929) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9483      +/-   ##
==========================================
- Coverage   90.85%   90.83%   -0.02%     
==========================================
  Files         719      731      +12     
  Lines       54235    54413     +178     
  Branches     8356     8394      +38     
==========================================
+ Hits        49276    49427     +151     
- Misses       3448     3455       +7     
- Partials     1511     1531      +20     

Impacted Files	Coverage Δ
...s/0026_alter_subscription_notification_and_more.py	100.00% <100.00%> (ø)
weblate/accounts/notifications.py	96.30% <100.00%> (+0.81%)	⬆️
weblate/accounts/tests/test_notifications.py	100.00% <100.00%> (ø)

... and 57 files with indirect coverage changes