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.
yep, we use too much ids in our selectors for my taste. I'm too bad at CSS (not enough experience) to _really_ know how to organize them, but I know enough to see when it's bad...