Log the following information in email headers:
* X-Danbooru-User: the user's name and ID.
* X-Danbooru-IP: the user's IP.
* X-Danbooru-Session: the users' session ID.
* X-Danbooru-URL: the page that triggered the email.
* X-Danbooru-Job-Id: the ID of the background job that sent the email.
* X-Danbooru-Enqueued-At: when the email was queued as a background job.
* X-Danbooru-Dmail: for Dmail notifications, the link to the Dmail.
* X-Request-Id: the request ID of the HTTP request that triggered the email.
Also make it so we log an event in the APM when we send an email.
Fix it so that emails are (hopefully) able to show the one-click
unsubscribe button in Gmail and other mail providers that support the
List-Unsubscribe header. This way users can unsubscribe instead of
marking emails as spam.
* Add the List-Unsubscribe-Post header.
* Fix the disable email notifications endpoint to support POST as well as DELETE requests.
* Fix the disable email notifications endpoint to disable XSRF protection (we don't need
users to be logged in because we use a signed URL instead).
https://www.rfc-editor.org/rfc/rfc8058#section-3.1https://www.rfc-editor.org/rfc/rfc8058#section-8.1
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.
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.
