Bug 1067228 - [Messages][Refactoring] Move subject management to a separate component. r=julien

I believe it's correct for ```compose_test``` tests as it shouldn't care about ```SubjectComposer``` internals (eg. if we didn't have to support subject input autogrow, I believe we should have used input instead of contenteditable), but yes in ```subject_composer_test``` we should use textContent and innerHTML. What do you think?