[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)
Sure this can't be solved with [#pluck](http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-pluck)?

Maybe
```ruby
current_user.contacts.sharing.joins(person: :profile).pluck(%w(contacts.id profiles.first_name profiles.last_name people.diaspora_handle))
```