Implement favourite tooltip to show who favourited a post

As per http://community.nodebb.org/topic/577/who-favorite-d-your-post.

Adds a `posts.favourited_users` field, easy for themers to use. Usernames are separated by comma. I've also added a favourite-tooltip class to easily select all the buttons that need a tooltip. Also for themers to use.

I swear I had a different implementation for `favourites.js` at first but then I thought "Why not copy their code, then I know for 100% that they'll accept it :D"