Prevent adding Crowd synced users to local groups in Stash

It isn't possible to add Crowd users to local Stash groups.
We should definitely prevent this in the UI or provide a warning that it isn't possible.

Comment from customer:

When trying to add a Crowd user, I click on the "Add user" button, and it just sits there... I see no errors (or new output) in the atlassian-stash.log file.