Add a new Owner user level for the site owner. Highly sensitive
operations like manually changing the passwords of other users will be
restricted to the site owner.
The old password reset flow:
* User requests a password reset.
* Danbooru generates a password reset nonce.
* Danbooru emails user a password reset confirmation link.
* User follows link to password reset confirmation page.
* The link contains a nonce authenticating the user.
* User confirms password reset.
* Danbooru resets user's password to a random string.
* Danbooru emails user their new password in plaintext.
The new password reset flow:
* User requests a password reset.
* Danbooru emails user a password reset link.
* User follows link to password edit page.
* The link contains a signed_user_id param authenticating the user.
* User changes their own password.
Few people used forum subscriptions (only around 100), and even fewer
people were subscribed to active threads. Most subscriptions were for
old threads that will never be bumped again. The implementation also had
a few problems:
* Unsubscribe links in emails didn't work (they unset the user's
receive_email_notifications flag, but forum subscriptions didn't
respect this flag).
* Some users had invalid email addresses, which caused notifications to
bounce. There was no mechanism for preventing bounces.
* The implementation wasn't scalable. It involved a daily linear scan
over _all_ forum subscriptions looking for any topics that had been updated.
