Allow the owner of a User Group to remove themselves from the group

Ref. GDPR19

Is your feature request related to a problem? Please describe.
As the owner (original creator) of a UserGroup I can’t leave it.

Describe the solution you’d like
I want to be able to remove myself if I’m not involved anymore with this UserGroup.

To not generate other problems an UserGroup would need to have always at least one administrator account.

Describe alternatives you’ve considered

Some related features but that are discarded for now as they’ll bring too much changes and we would need to have even more features (like a better Groups management in the admin):

• To have a way to delete a UserGroup if there’s anyone else.
• To have a “Danger Zone” section with this kind of actions.
• To change the ownership of these “orphan” UserGroup to an administrator, but it’d be weird to see that an admin has lots of UserGroups. It could be seen as “this person is astroturfing”.

Does this issue could impact on users private data?
No, the groups members are public.

Acceptance criteria

• Given that I’m the original creator of a UserGroup
  When I see my role in it
  Then it doesn’t says that I’m the owner
  And it says that I’m an administrator
• Given that I’m the original creator of a UserGroup
  And there’s at least one other administrator in it
  I can remove myself from it
• Given that I’m the original creator of a UserGroup
  And there isn’t any other administrator in it
  Then I can’t remove myself from it
  And I see a callout that says “you can’t remove yourself from this group as you’re the last administrator”