This commit adds a new notification that gets sent to admins when the site gets new features after an upgrade/deploy. Clicking on the notification takes the admin to the admin dashboard at `/admin` where they can see the new features under the "New Features" section.
Internal topic: t/87166.
This PR moves the behavior from the PostAlerter. We delete an existing liked notification and set the `username2` attribute to the previous `display_username`. We repeat this process unless the last one is old enough or it's not in the most recent ones.
We previously used ConsolidateNotifications with a threshold of 1 to re-use an existing notification and bump it to the top instead of creating a new one. It produces some jumpiness in the user notification list, and it relies on updating the `created_at` attribute, which is a bit hacky.
As a better alternative, we're introducing a new plan that deletes all the previous versions of the notification, then creates a new one.
Osama Sayegh
Roman Rizzi