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/assets/javascripts/discourse/app/controllers/preferences/notifications.js

@@ -9,7 +9,6 @@ export default Controller.extend({
 
     this.saveAttrNames = [
       "muted_usernames",
-      "ignored_usernames",
       "new_topic_duration_minutes",
       "auto_track_topics_after_msecs",
       "notification_level_when_replying",

app/assets/javascripts/discourse/app/controllers/preferences/users.js

@@ -43,7 +43,6 @@ export default Controller.extend({
 
     this.saveAttrNames = [
       "muted_usernames",
-      "ignored_usernames",
       "allowed_pm_usernames",
       "enable_allowed_pm_users",
     ];