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.

test/mailers/previews/user_mailer_preview.rb

@@ -3,4 +3,9 @@ class UserMailerPreview < ActionMailer::Preview
     dmail = User.admins.first.dmails.first
     UserMailer.dmail_notice(dmail)
   end
+
+  def password_reset
+    user = User.find(params[:id])
+    UserMailer.password_reset(user)
+  end
 end