auth: add project to watched on accepting an invitation

[nijel]

Proposed changes

This is expected for people to follow project once they accept an invitation. See #5155

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 #9997 (4c225e7) into main (9b74c90) will increase coverage by 0.00%.
Report is 28 commits behind head on main.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #9997   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         533      533           
  Lines       55307    55309    +2     
  Branches     8781     8782    +1     
=======================================
+ Hits        50293    50298    +5     
+ Misses       3483     3479    -4     
- Partials     1531     1532    +1     

Files Changed	Coverage
weblate/auth/views.py	0.00%