Handle removal of user from last group. See #11875

Conflicts:

	components/tools/OmeroWeb/omeroweb/webadmin/views.py
Fixes and docs from PR #1935

Conflicts:

	components/tools/OmeroWeb/omeroweb/webadmin/views.py
setMembersOfGroup returns ExperimenterWrapper list
Improve error message for removing users
Merge pull request #1980 from will-moore/remove_user_last_group_11875_develop

Remove user last group 11875 develop