Remove unused $roows variable.
Add hooks to support injecting custom fields into the registration form.
Check `$_REQUEST` for `tix_tickets_selected` instead of `$_POST`.

This is so we can also support a workflow where the ticket data is passed in via `$_GET`, which will be necessary
for the upcoming require-login addon.
Support safe HTML in admin notices.
Introduce required-login addon.

props @NowellVanHoesen
see https://meta.trac.wordpress.org/ticket/441