FIX: respect moderator group permissions in guardian (#10713)

Since 9e4ed03, moderators can view groups with visibility level set to "Group owners, members and moderators".

This fixes an issue where moderators can see the group in /g but then get a 404 when clicking on individual groups.

spec/requests/groups_controller_spec.rb

@@ -845,7 +845,7 @@ describe GroupsController do
       end
 
       it 'should not be able to update a group it cannot see' do
-        group.update!(visibility_level: 2)
+        group.update!(visibility_level: Group.visibility_levels[:owners])
 
         put "/groups/#{group.id}.json", params: { group: { name: 'testing' } }