Make private favgroups a Gold-only option. This is for consistency with
private favorites and upvotes being Gold-only options.
Existing Members with private favgroups are allowed to keep them, as
long as they don't disable privacy. If they disable it, then they can't
re-enable it again without upgrading to Gold first.
* Fix it so that all edit forms show an error banner if the form
has validation errors. Previously forms had to manually call
`error_messages_for`, which not all forms did.
* Fix it so that the full validation error message is shown next to each
input attribute that had errors. Also update the styling of these
error messages to look better.
