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.
Because of that rule: 
```
section.new[role="region"] .article-list[data-type="list"] li:first-child {
    margin-top: 4.6rem;
}
```