AO3-7156 Add scheduled task to delete unused admin post tags

[pmonfort]

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Issue

https://otwarchive.atlassian.net/browse/AO3-7156

Purpose

Adds a weekly scheduled Resque job that automatically deletes admin post tags not used on any admin post. The logic is extracted into AdminPostTag.delete_unused, which the existing rake task (Tag:delete_unused_admin_post_tags) now delegates to as well.

Testing Instructions

1. Create some admin post tags (via admin posts in the admin panel)
2. Remove the tags from the posts so they become unused
3. Run rake Tag:delete_unused_admin_post_tags from the console
4. Verify that only unused tags are deleted

References

The existing rake task used raw SQL for the LEFT JOIN. This PR replaces it with left_outer_joins, which is more idiomatic Rails.

Credit

Pablo Monfort (he/him)