mixins: reuse url path for full_slug

[nijel]

Proposed changes

These can be same so reduce code duplication and use existing url path.

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 #9753 (0e89d10) into main (5899ef1) will increase coverage by 0.09%.
Report is 17 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9753      +/-   ##
==========================================
+ Coverage   90.75%   90.84%   +0.09%     
==========================================
  Files         758      768      +10     
  Lines       56065    56191     +126     
  Branches     8719     8741      +22     
==========================================
+ Hits        50883    51048     +165     
+ Misses       3635     3601      -34     
+ Partials     1547     1542       -5     

Files Changed	Coverage Δ
weblate/trans/models/component.py	80.74% <ø> (-0.04%)	⬇️
weblate/trans/models/translation.py	87.32% <ø> (-0.05%)	⬇️
weblate/trans/models/unit.py	86.50% <ø> (+0.07%)	⬆️
weblate/trans/mixins.py	96.96% <100.00%> (+0.04%)	⬆️
weblate/trans/tests/test_manage.py	98.58% <100.00%> (ø)
weblate/utils/stats.py	92.56% <100.00%> (-0.03%)	⬇️

... and 32 files with indirect coverage changes