Bug 1021788 - [Messages] Can't click on the suggestion list when there are a lot of recipients in the recipients panel

- Set fix height for subheader under the new compose view
We could not lock the height of ```#messages-to-field``` now because the old recipient container was built by ```#messages-recipients-list-container```, not outer container ```#messages-to-field```. So setting ```#messages-to-field``` height to 0 is fine at that time. Now the container is built by ```#messages-to-field```, so only changing the height of container just break the recipient container directly. I think setting the subheader height to 0 is just like the same way we did before.