[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)
<code>Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '["contacts.id", "profiles.first_name", "profiles.last_name", "people.diaspora_ha' at line 1: SELECT ["contacts.id", "profiles.first_name", "profiles.last_name", "people.diaspora_handle"] FROM `contacts` INNER JOIN `people` ON `people`.`id` = `contacts`.`person_id` INNER JOIN `profiles` ON `profiles`.`person_id` = `people`.`id` WHERE `contacts`.`user_id` = 1 AND `contacts`.`sharing` = 1</code>