Show "Leave group" option for open and restricted groups that user is a member of

[robertknight]

Previously the "Leave group" option was only shown for private groups on the basis that membership of open and restricted groups was managed by h admins. However first-party Hypothesis users can now create open and restricted groups themselves and members of these groups will see an option to leave the group on activity pages. Align whether the client shows the "Leave group" option with h's behavior.

During this change it was noticed that the documentation in the code and test descriptions did not match the actual handling of allowLeavingGroups. The comments said that services had to explicitly set allowLeavingGroups to prevent users leaving. However the code would treat allowLeavingGroups as false if a) service configuration was present and b) the value of allowLeavingGroups was falsey (including undefined). Changing this behavior may cause issues for existing users of third party authorities, so this commit updates the documentation and tests to accurately describe the current behavior.

Fixes #6637 (Slack thread)

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Testing:

1. Create an open group at http://localhost:5000/groups/new
2. Verify that you see the "Leave group" option on activity pages
3. On the main branch in the client, the new group will appear in the drop-down menu but the "Leave group" option will be missing
4. On this branch the "Leave group" option should be present and work when clicked

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (67b74ce) to head (600b3fe).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6638      +/-   ##
==========================================
- Coverage   99.42%   99.42%   -0.01%     
==========================================
  Files         270      270              
  Lines       10174    10172       -2     
  Branches     2419     2418       -1     
==========================================
- Hits        10116    10114       -2     
  Misses         58       58              

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[acelaya]

LGTM 👍🏼

I tested it and works as expected. In main branch I did not see the option to leave group for non-private groups, but I do see it in this branch.