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/controllers/maintenance/user/password_resets_controller.rb

@@ -1,38 +0,0 @@
-module Maintenance
-  module User
-    class PasswordResetsController < ApplicationController
-      def new
-        @nonce = UserPasswordResetNonce.new
-      end
-
-      def create
-        @nonce = UserPasswordResetNonce.create(nonce_params)
-        if @nonce.errors.any?
-          redirect_to new_maintenance_user_password_reset_path, :notice => @nonce.errors.full_messages.join("; ")
-        else
-          redirect_to new_maintenance_user_password_reset_path, :notice => "Email request sent"
-        end
-      end
-
-      def edit
-        @nonce = UserPasswordResetNonce.where(:email => params[:email], :key => params[:key]).first
-      end
-
-      def update
-        @nonce = UserPasswordResetNonce.where(:email => params[:email], :key => params[:key]).first
-
-        if @nonce
-          @nonce.reset_user!
-          @nonce.destroy
-          redirect_to new_maintenance_user_password_reset_path, :notice => "Password reset; email delivered with new password"
-        else
-          redirect_to new_maintenance_user_password_reset_path, :notice => "Invalid key"
-        end
-      end
-
-      def nonce_params
-        params.fetch(:nonce, {}).permit([:email])
-      end
-    end
-  end
-end