Bug 949457 - Make Compose into a flex layout r=julienw

This patch is removing all the logic related with the input height, adding a flex layout. Things to take into account:
- This is a new approach being validated by UX. In this case, subject & body shares the same 'container', and you can use the scroll
- Im keeping the same namespace for not adding a massive change, but we should called everything related with the composer as `composer-` instead of `messages-` when splitting this.