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

the goal of using DOM properties to set data (textContent and innerHTML) is to simulate the user typing the data, to see how the code behaves in that regard, as opposed to setSubject which is programmatic.

I think we should continue using textContent and innerHTML to test this. You can have a separate test for setSubject to check it does the correct thing to the DOM of course.