Add announcement for category page

[harriebird]

Proposed changes

Add announcement form on category page.
To achieve this, changes are made to the models Change and Announcement.
This solves issue #11106.

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

Attention: Patch coverage is 82.60870% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 90.57%. Comparing base (6aee586) to head (33cf2f7).
Report is 1850 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11644      +/-   ##
==========================================
- Coverage   90.82%   90.57%   -0.26%     
==========================================
  Files         554      573      +19     
  Lines       57306    58680    +1374     
  Branches     9122     9373     +251     
==========================================
+ Hits        52046    53147    +1101     
- Misses       3640     3843     +203     
- Partials     1620     1690      +70     

Files	Coverage Δ
weblate/trans/forms.py	87.76% <100.00%> (-0.10%)	⬇️
..._announcement_severity_change_category_and_more.py	100.00% <100.00%> (ø)
weblate/trans/models/announcement.py	89.58% <100.00%> (+0.22%)	⬆️
weblate/trans/templatetags/translations.py	89.51% <ø> (+0.30%)	⬆️
weblate/trans/tests/test_manage.py	98.67% <100.00%> (+0.09%)	⬆️
weblate/trans/views/basic.py	82.73% <ø> (-0.20%)	⬇️
weblate/wladmin/tests.py	100.00% <100.00%> (ø)
weblate/trans/models/change.py	81.66% <33.33%> (+2.65%)	⬆️

... and 248 files with indirect coverage changes

[nijel]

Scheduled for merge, thanks for your contribution!

[emmapeel2]

I have tried this new feature, but when I post an announcement to the Tor Browser category at https://hosted.weblate.org/projects/tor/tor-browser/ , the announcement is displayed in the whole project, not only on the category.