[WIP] Display new conversation form on conversations/index

Fixes #4114, might also fix #5177. Should be already fully functional. Needs some style fixes and some new/adapted tests.

![](https://cloud.githubusercontent.com/assets/3798614/4090139/350e44f8-2f79-11e4-9753-8fa231dca851.png)
Maybe pull this to the beginning of the method to save the expensive stuff for the views

``ruby
if !params[:facebox] && request.format.html?
  redirect_to conversations_path
  return
end