Prevent the sole owner from leaving a group

[seanh]

See also: #9147

From the Engineering Requirements Document: If you are the sole owner of a group, you cannot leave that group without designating a new owner–unless it has no annotations other than your own.

From the Product Requirements Document: if the group has other Members and especially other Users’ annotations I will need to elevate a group Member or Moderator to take the Owner role over as I leave.

Note that preventing the sole owner from leaving a group cannot remove the possibility of owner-less groups existing: there are already owner-less groups in the production DB, and more can be created if the sole owner of a group deletes their account or if a Hypothesis admin deletes their account. So h code and UI design will have to continue to account for the possibility of groups with no owners.

I think we may want to allow the sole owner to leave a group if:

1. The group doesn't contain any annotations from other users, even if the group does have other members.
2. The group doesn't have any other members, even if the group does contain annotations from other users.

In all other cases if the last owner tries to leave a group they should see an error message telling them that they need to change another user's role to owner first. The error message should contain a link to the https://hypothes.is/groups/{pubid}/edit/members page where the user can make this role change.

Note that there are multiple ways that a user can attempt to leave a group, and we need to cover all of them:

1. A user can call the remove-member-from-group API directly: https://h.readthedocs.io/en/latest/api-reference/v1/#tag/groups/paths/~1groups~1{id}~1members~1{user}/delete

2. A user can use the Leave group button in the client's groups menu:

   

3. A user can use the Leave this group button in the sidebar on the group's page (https://hypothes.is/groups/{pubid}/flag-test):

   

4. (Not yet implemented:) a user can delete themselves from the group using the Members table on the group's https://hypothes.is/groups/{pubid}/edit/members page. This is not yet implemented: at the time of writing the user can't remove themselves from the group on this page. But there's a plan to enable it at some point.