Manage replies on the client side

The 'reply' link associated with a comment now carries some information about the comment which is used to generate the reply form. No need to make a callback to the server.

I did not delete the old reply controller code but I think it should go away.

This is a WIP, extracted from a project so there may be some rough edges, like I wrote the HAML templates without testing them (scandal!) and we may want to improve the reply template styling (it is currently done in my client app). 

Tell me what you think!