Fix mailers to not attempt deliveries to invalid or nonexistent email
addresses. This usually happened when someone changed their email, and
we tried to send a confirmation email to a nonexistent address.
* Add README files to several directories in app/ giving a brief
overview of some parts of Danbooru's architecture.
* Add documentation for files in config/.
* 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.
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.
Remove the login reminder page. The meaning of "login reminder" wasn't
clear (it's for recovering a forgotten username) and the functionality
was redundant. The password reset page can already be used to recover
forgotten usernames.
There was also a privacy leak, since the login reminder page could be
used to find out whether a given email is in use on Danbooru.
