Confirmable user invitations

[nijel]

Proposed changes

• Make invitations standalone model
• Do not start social auth flow when sending an invitation
• List user invitations to the user profile
• Merge all e-mail invitations upon registering on a site

Closing #9261 and #9131

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 #9566 (02bb1da) into main (0ab1430) will decrease coverage by 0.06%.
Report is 3 commits behind head on main.
The diff coverage is 78.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9566      +/-   ##
==========================================
- Coverage   91.06%   91.00%   -0.06%     
==========================================
  Files         760      764       +4     
  Lines       56161    56271     +110     
  Branches     8732     8747      +15     
==========================================
+ Hits        51141    51208      +67     
- Misses       3492     3527      +35     
- Partials     1528     1536       +8     

Files Changed	Coverage Δ
weblate/accounts/models.py	88.61% <ø> (ø)
weblate/auth/tasks.py	0.00% <0.00%> (ø)
weblate/trans/forms.py	87.79% <ø> (+0.20%)	⬆️
weblate/urls.py	81.70% <ø> (ø)
weblate/trans/models/change.py	80.28% <16.66%> (-0.64%)	⬇️
weblate/auth/views.py	67.15% <45.09%> (-13.75%)	⬇️
weblate/accounts/pipeline.py	81.78% <78.57%> (-0.76%)	⬇️
weblate/utils/forms.py	91.83% <80.00%> (-4.06%)	⬇️
weblate/auth/models.py	92.24% <80.76%> (-0.67%)	⬇️
weblate/accounts/views.py	83.76% <90.47%> (-0.12%)	⬇️
... and 10 more

... and 2 files with indirect coverage changes