[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)
mmh, missing splat: `.pluck(*%w(`

If you copied that code maybe it's time to move to private controller method, a helper method or even introduce a Presenter.