Remove from all conversations when leaving group

[nicksellen]

Fixes karrot-dev/karrot-frontend#2571

There are multiple types of chat where users are not always removed from the chat when leaving the group. Activity chats is one case, which was an issue for past activity chats.

[codecov]

Codecov Report

Merging #1264 (1997dfd) into master (cc200e1) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1264      +/-   ##
==========================================
+ Coverage   92.87%   92.92%   +0.04%     
==========================================
  Files         537      539       +2     
  Lines       21431    21545     +114     
  Branches     1547     1549       +2     
==========================================
+ Hits        19904    20020     +116     
+ Misses       1208     1207       -1     
+ Partials      319      318       -1     

Files Changed	Coverage Δ
karrot/activities/receivers.py	98.64% <100.00%> (ø)
...rations/0043__cleanup_conversation_participants.py	100.00% <100.00%> (ø)
karrot/conversations/models.py	97.67% <100.00%> (+0.01%)	⬆️
karrot/conversations/tests/test_api.py	100.00% <100.00%> (ø)
karrot/conversations/tests/test_data_migrations.py	100.00% <100.00%> (ø)
karrot/groups/receivers.py	89.58% <100.00%> (+0.92%)	⬆️

... and 2 files with indirect coverage changes

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[tiltec]

Seems good! I guess we can make a database backup right before deploying to production, just to be safe :) Also, messages are not lost when a participant is deleted, so it's mostly reversible.