Automatically disable public sign up or at least give warning when public sign-up is enabled and crowd is used

In cases where the only authentication / user database is crowd (which is read-only), the public sign up check box should be greyed out or automatically unchecked.

Additionally, a warning should appear when public sign-up is enabled and then later crowd is added and the internal directory is disabled.

Currently, it appears to allow enabling public signup when crowd is the only enabled authentication database. Trying to add a user in this case generates an error (attached screenshot).