chore: refactor antispam API

[nijel]

Proposed changes

Allow future extension to services supporting checking multiple strings at once.

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 85.29412% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.92%. Comparing base (3ec13a8) to head (250a65b).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/utils/antispam.py	78.57%	3 Missing ⚠️
weblate/trans/forms.py	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #13187       +/-   ##
===========================================
+ Coverage   23.97%   90.92%   +66.94%     
===========================================
  Files         601      601               
  Lines       62156    62160        +4     
  Branches     6406     6409        +3     
===========================================
+ Hits        14905    56519    +41614     
+ Misses      47115     3978    -43137     
- Partials      136     1663     +1527     

Files with missing lines	Coverage Δ
weblate/trans/views/edit.py	81.26% <ø> (+64.35%)	⬆️
weblate/utils/tests/test_antispam.py	96.36% <100.00%> (+96.36%)	⬆️
weblate/trans/forms.py	89.74% <77.77%> (+52.03%)	⬆️
weblate/utils/antispam.py	92.85% <78.57%> (+66.54%)	⬆️

... and 452 files with indirect coverage changes