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
ok, I see it breaks the container when we set "height: 0" on the to-field

I wish `.new .subheader` would have been enough (or even simply `.subheader`). But let's see this when we'll simplify the CSS later