Bug 1034600 - [Messages] Suggestions list is badly located. r=julien

* Recipient suggestions container moved to more appropriate place;
* Recipient suggestions now occupy all possible space minus heights of the app header and message "To" field.
* ```.contact-list``` styles are cleaned up a bit.
I know I always did like this (related to the markup-for-short-lived-elements discussion we had earlier) for simplicity sake, but I also know it's a habit so I'm happy to revisit this if we have evidence of either too much hold memory with event handlers or event setting/removing taking time (or not).

In the mean time, let's say it's a style habit and stick to the 'less code' rule. Do you agree?