[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)
Meh, you always see something. Should've used array decomposition here:
```ruby
.map {|contact_id, *name_attrs|
  {value: contact_id, name: Person.name_from_attrs(*name_attrs) }
}