[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 gsub is a simple `.tr(%("), %('))` (percent literals for better readability). I'd also format the map a bit differently:
```ruby
  .pluck(..)
  .map {|contact|
     # why not id as key?         # actually why the replacement? to_json should handle `"` in values perfectly fine.
    {value: contact[0], name: Person.name_from_attrs(*contact.last(3)).tr(%["], %['])}
  }
  .to_json