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

spec/requests/post_action_users_controller_spec.rb

@@ -64,7 +64,7 @@ describe PostActionUsersController do
     PostActionCreator.like(ignored_user, post)
     regular_user = Fabricate(:user)
     PostActionCreator.like(regular_user, post)
-    IgnoredUser.create(user: user, ignored_user: ignored_user)
+    Fabricate(:ignored_user, user: user, ignored_user: ignored_user)
 
     get "/post_action_users.json", params: {
       id: post.id, post_action_type_id: PostActionType.types[:like]