[WIP] Followers

This is a feature branch for following people. It includes:

* The ability to follow and unfollow people
* Automatic email notifications for followers when posting a new listing
* A "people you follow" grid in the profile page
This will be executed in each and every page.

Add this to own named function and call that function from the page that uses to follow button. Put that function call inside `content_for extra_javascript`

And no need to `$(document).ready` since jQuery is already loaded when the extra javascript content is loaded.