Check for removing last owner

Issue #720
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/views/acl.py

@@ -68,7 +68,10 @@ def revoke_owner(request, project):
     obj, form = check_user_form(request, project)
 
     if form is not None:
-        pass
+        if obj.owners.count() <= 1:
+            messages.error(request, _('You can not remove last owner!'))
+        else:
+            pass
 
     return redirect_param(
         'project',