[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)
That doesn't work either. I get no error anymore but
<code>current_user.contacts.sharing.joins(person: :profile).pluck(*%w(contacts.id profiles.first_name profiles.last_name people.diaspora_handle)).count(:all)</code> is 0.

I moved the code to the private controller method contacts_json.