* 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.
* Make IP bans soft deletable.
* Add a hit counter to track how many times an IP ban has blocked someone.
* Add a last hit timestamp to track when the IP ban last blocked someone.
* Add a new type of IP ban, the signup ban. Signup bans restrict new
signups from editing anything until they've verified their email
address.
