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.
Initially I was thinking about single container for the ```messages-to-field``` and this new section because it did not look really easy to add this to the ```messages-to-field```, because of the fact that it's flexbox + using transform translate to become multiline - so I was afraid regressions here.

Now when I know that ```vh``` isn't an option (and I do agree with it after reading the performance bug you've filed), I think this node will become child of the main panel element to able to use % based height. Please let me know if you see other options!