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 https://github.com/hypothesis/client/issues/6637