users: refactor password reset flow.

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.

app/mailers/application_mailer.rb

@@ -0,0 +1,3 @@
+class ApplicationMailer < ActionMailer::Base
+  default from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.contact_email}>", content_type: "text/html"
+end