Modernize AvailableForumsActivity

Turn list of available forums into a `BriarRecyclerView` with XML layout.
Allow to respond to forum invitations from the list of available forums.
The user can either accept or decline an invitation.

![device-2016-05-03-141150](/uploads/0998877d47fb7a3eafe3a90f917d8716/device-2016-05-03-141150.png)

See merge request !172