users: allow mods to change the names of other users.

Allow moderators to forcibly change the username of other users. This is
so mods can change abusive or invalid usernames.

* A mod can only change the username of Builder-level users and below.
* The user can't change their own name again until one week has passed.
* A modaction is logged when a mod changes a user's name.
* A dmail is sent to the user notifying them of the change.
* The dmail does not send the user an email notification. This is so we
  don't spam users if their name is changed after they're banned, or if
  they haven't visited the site in a long time.

The rename button is on the user's profile page, and when you hover over
the user's name and open the "..." menu.

app/models/dmail.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class Dmail < ApplicationRecord
-  attr_accessor :creator_ip_addr
+  attr_accessor :creator_ip_addr, :disable_email_notifications
 
   validate :validate_sender_is_not_limited, on: :create
   validates :title, presence: true, length: { maximum: 200 }, if: :title_changed?
@@ -145,7 +145,7 @@ class Dmail < ApplicationRecord
   end
 
   def send_email
-    if is_recipient? && !is_deleted? && to.receive_email_notifications?
+    if is_recipient? && !is_deleted? && to.receive_email_notifications? && !disable_email_notifications
       UserMailer.with(headers: { "X-Danbooru-Dmail": Routes.dmail_url(self) }).dmail_notice(self).deliver_later
     end
   end