DEV: Cleanup ignored user logic (#11107)

- IgnoredUser records should all now have an expiring_at value. This commit enforces that in the DB, and fixes any corrupt rows
- Changes to the ignored user list are now handled by the `/u/{username}/notification_level` endpoint. This allows setting expiration dates on the ignore. This commit removes the old logic for saving a list of usernames in the user preferences.
- Many specs were calling `IgnoredUser.create`. This commit changes them to use `Fabricate(:ignored_user)` for consistency

app/controllers/users_controller.rb

@@ -1586,7 +1586,6 @@ class UsersController < ApplicationController
       :title,
       :date_of_birth,
       :muted_usernames,
-      :ignored_usernames,
       :allowed_pm_usernames,
       :theme_ids,
       :locale,