A follow link for Manage Posts, Calendar, and Story Budget

This adds a "Follow" link to post row actions to Manage Posts, Calendar, and Story budget views. It also removes the unfollowing_users taxonomy, as that hasn't ever been used, and refactors follow_post_users and unfollow_post_users(), as the latter hasn't ever worked.

Lastly, it refactors the row actions for the Story Budget so it's easy to hook on this follow link

See #130
Linked features from dashboard
Proper use of sprintf, and consolidate some code
Props for #185
A different way of binding events, which means the event is dynamically bound instead of static.
Merge pull request #185 from Automattic/follow-link

A follow link for Manage Posts, Calendar, and Story Budget