* Move emails from users table to email_addresses table.
* Validate that addresses are formatted correctly and are unique across
users. Existing invalid emails are grandfathered in.
* Add is_verified flag (the address has been confirmed by the user).
* Add is_deliverable flag (an undeliverable address is an address that bounces).
* Normalize addresses to prevent registering multiple accounts with the
same email address (using tricks like Gmail's plus addressing).
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.
