feat(notifications): add notification on strings needing updating

[nijel]

Proposed changes

I was originally considering a separate notification for this, but at last I think that having just one here is a better approach.

Fixes #10651

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

All modified and coverable lines are covered by tests ✅

Project coverage is 90.61%. Comparing base (6aee586) to head (e0c37a9).
Report is 1782 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11640      +/-   ##
==========================================
- Coverage   90.82%   90.61%   -0.21%     
==========================================
  Files         554      571      +17     
  Lines       57306    58648    +1342     
  Branches     9122     9372     +250     
==========================================
+ Hits        52046    53146    +1100     
- Misses       3640     3815     +175     
- Partials     1620     1687      +67     

Files	Coverage Δ
...migrations/0010_alter_subscription_notification.py	100.00% <100.00%> (ø)
weblate/accounts/notifications.py	95.74% <100.00%> (+0.02%)	⬆️
weblate/accounts/tests/test_notifications.py	100.00% <ø> (ø)

... and 249 files with indirect coverage changes

[laoneo]

So when a string in the souorce file is updated, what happens with the translations? Do they get deleted or is it just a notification? Transifex for example (where I come from) deletes all the translations. This makes very much sense to me as the translations do not reflect anymore the current source string.