Moves row height into a const.
Caches header views, provides a way to update title.
Updates for removing/updating followed/unfollowed posts.

Adds a way to update the following status for all posts belonging to a particular site.
Adds a way to delete posts belonging to a specific site from a specific topic
Updates the post merge cleanup logic to use the startDate of a batch of posts vs the date of the earliest post. This catches some stale posts that would other wise remain.
Adds a method to retrive the 'sites I follow' topic.
Improves follow/unfollow site logic.

When following/unfollowing a site via the subscription feature, the "sites I follow" topic, if present,  will refresh in the background.
Updates the way the section header is update as sites are followed/removed.