Added in remaining metadata for Threads. Removed username from post meta

* Added last_post_username, last_post_created_at, and view_count
* View count is set to 0 and never changes currently once jwt is
implemented then we can make view count functional.
* Removed username metadata field from post (will query, via frontend,
the user data using post.user_id)
* Fixed keys test to work with new metadata fields and removed username
field
* Added tests to metadata test for new Threads metadata