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.
re: adding to messages-to-field: the transformation will make things difficult, you're right, so forget it.

re: additional container: yeah, this comes from the fact we don't use BEM-like style... so maybe we don't have any choice for this... In this case we clearly have a technical debt regarding CSS in my opinion, but this is not the scope of this bug anyway.