Posts: Non-trashed posts should take slug priority over trashed posts.
When determining a unique post slug, trashed posts are taken into account. Previously, new posts would add suffixes to their slugs (e.g. about-2) when a post in the trash had the desired slug (e.g. about).
To avoid this behavior, when a post is trashed its slug (i.e. post_name) is now suffixed with -%trashed%. The post's pre-trash slug is stored as post meta, and if the post is restored from trash, its desired slug is reapplied.
For existing trashed posts which don't have the -%trashed% suffix, the suffix is added when a post with its desired slug is created.
Props ocean90, boonebgorges, ryan, SergeyBiryukov, coffee2code, helen, williamsba1.
See #11863.