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/models/ignored_user.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class IgnoredUser < ActiveRecord::Base
+  validates :expiring_at, presence: true
+
   belongs_to :user
   belongs_to :ignored_user, class_name: "User"
 end