fixes https://github.com/TryGhost/Team/issues/2919
This pull request implements a new feature that allows bulk editing of
posts by a filter. It adds a new `bulkEdit` endpoint to the posts API
and new `PostsService` methods to handle the bulk actions.
The posts list component is duplicated, so we can keep working in a
copied version without affecting the old version without a flag. It
temporarily adds a star icon to indicate featured posts in the posts
list.
