Fixed user email auto verification on accepting organization invitation

[klakhov]

Motivation and context

There is a couple of UX bugs in invite user to organization feature. This pr fixes:

• Email is auto-verified after accepting invitation
• Stuff can view unaccepted invitations
• Stuff can edit unaccepted memberships
• User email is now used as username

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)
• I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov]

Codecov Report

Merging #7073 (5f0f517) into develop (ba3db01) will increase coverage by 0.03%.
Report is 3 commits behind head on develop.
The diff coverage is 44.44%.

@@             Coverage Diff             @@
##           develop    #7073      +/-   ##
===========================================
+ Coverage    82.31%   82.35%   +0.03%     
===========================================
  Files          361      361              
  Lines        39263    39270       +7     
  Branches      3588     3588              
===========================================
+ Hits         32321    32340      +19     
+ Misses        6942     6930      -12     

Components	Coverage Δ
cvat-ui	77.37% <ø> (+0.07%)	⬆️
cvat-server	86.91% <44.44%> (-0.02%)	⬇️