fix(member merge): API updates for frontend

[mifu67]

• Use slug instead of org name
• Use camel case instead of snake case for parameters
• Return serialized merged user in POST request (so we can update query data)
• Use set instead of list for orgs, because users whose accounts have been previously merged sometimes have multiple org member objects for the same org. This might just be temporary, but in any case a set is not worse than a list ¯\_(ツ)_/¯

[cathteng]

You can make AuthMergeUserAccountsValidator subclass CamelSnakeSerializer and keep the snake case here, we like snake case on the backend

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/users/api/serializers/user.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #96709       +/-   ##
===========================================
+ Coverage   61.21%   80.06%   +18.84%     
===========================================
  Files        8505     8484       -21     
  Lines      374455   379694     +5239     
  Branches    24301    24179      -122     
===========================================
+ Hits       229230   303999    +74769     
+ Misses     142732    75318    -67414     
+ Partials     2493      377     -2116