users: delete more data when user deactivates their account.

* Don't delete the user's favorites unless private favorites are enabled. The general rule is that
  public account activity is kept and private account activity is deleted.
* Delete the user's API keys, forum topics visits, private favgroups, downvotes, and upvotes (if
  privacy is enabled).
* Reset all of the user's account settings to default. This means custom CSS is deleted, where it
  wasn't before.
* Delete everything but the user's name and password asynchronously.
* Don't log the current user out if it's the owner deleting another user's account.
* Fix #5067 (Mod actions sometimes not created for user deletions) by wrapping the deletion process
  in a transaction.

app/jobs/delete_user_job.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+# A job that deletes a user's settings and other personal data when they delete their account.
+class DeleteUserJob < ApplicationJob
+  queue_as :default
+  queue_with_priority 20
+
+  def perform(user)
+    UserDeletion.new(user: user).delete_user
+  end
+end