Bug 906753 - [Messages] we should see the recipients count in the header when composing a multi-recipient messag

https://bugzilla.mozilla.org/show_bug.cgi?id=906753

- remove call to l10n.get
- replaced to use updateComposerHeader (per Julien W's recommendation)
- tests
    - add recipients updates composer header
    - remove recipients updates composer header
- updated mock recipients to have basic recipient remove logic