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

I'm not very fond of the naming "members" and "privateMembers". In recipients.js we have `priv` for the map, and we use something like `var view = priv.get(this)` because we manipulate the view. Here we could use either `var subject = priv.get(this)` or `var that = priv.get(this)`. Or keep `privateMembers` and use `var priv = privateMembers.get(this)`. What do you think?