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.

db/migrate/20200309035334_drop_user_password_reset_nonces.rb

@@ -0,0 +1,7 @@
+require_relative "20110717010705_create_user_password_reset_nonces"
+
+class DropUserPasswordResetNonces < ActiveRecord::Migration[6.0]
+  def change
+    revert CreateUserPasswordResetNonces
+  end
+end