Add description to Label and put a toolip

[harriebird]

Proposed changes

Added description field to the Label model and set it as a tooltip.
This solves issue #9227.

Example images:

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

[nijel]

• Seems like the tests were not updated to new field.
• LabelSerializer for the API needs update as well.

[harriebird]

Updated the tests and added description in the LabelSerializer fields.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.61%. Comparing base (6aee586) to head (dbe333c).
Report is 4380 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11907      +/-   ##
==========================================
- Coverage   90.82%   90.61%   -0.21%     
==========================================
  Files         554      578      +24     
  Lines       57306    58835    +1529     
  Branches     9122     9393     +271     
==========================================
+ Hits        52046    53313    +1267     
- Misses       3640     3829     +189     
- Partials     1620     1693      +73     

Files with missing lines	Coverage Δ
weblate/api/serializers.py	92.57% <100.00%> (+0.10%)	⬆️
weblate/api/tests.py	100.00% <100.00%> (ø)
weblate/trans/forms.py	88.55% <100.00%> (+0.68%)	⬆️
weblate/trans/migrations/0019_label_description.py	100.00% <100.00%> (ø)
weblate/trans/models/label.py	100.00% <100.00%> (ø)
weblate/trans/tests/test_labels.py	100.00% <100.00%> (ø)

... and 329 files with indirect coverage changes

[nijel]

Merged, thanks for your contribution!