* Make authentication methods into User instance methods instead of
class methods.
* Fix API key authentication to use a secure string comparison. Fixes a
hypothetical (unlikely to be exploitable) timing attack.
* Move login logic from SessionCreator to SessionLoader.
Pull the password reauthentication logic out of the user model and put
it in the password update controller where it belongs.
This fixes an issue where when a new user was created the user model had
an incorrect password error set on it by `encrypt_password_on_update`.
It was trying to verify the old password even though we don't have one
when creating a new user. This error caused the user create action to
redirect back to the signup page because `respond_with` thought that
creating the user failed.
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.
* Fix users being redirected back to the change password page after
successfully changing their password.
* Move passwords controller out of /maintenance/ namespace.
* Add tests.
