Bug 1087933 - Allow to add recipients to a message being written in a thread

We redirect to the Composer with current subject, text, attachments and
contacts.
can't we extend the argument we use when we call toPanel? For example:
```javascript
Navigation.toPanel('composer', {
  composer: Compose, // in the future, we'll have a specific instance (see https://bugzilla.mozilla.org/show_bug.cgi?id=1011085 and dependencies)
  recipients: Threads.active.participants // in the future, I expect that ThreadUI.recipients works in both thread panel view and composer view
});
```

Then, in beforeEnterComposer you could handle these arguments. With the current code, you could just do nothing with Compose because it would stay the same (and in the future we could have some code to copy the content, in Compose.js). With the recipients part I think we'd need to do what you do here, but you can try to factorize it with what we do in handleActivity because this is actually the same thing.

I think you'd need to add `Threads.currentId = null;` in a `if (Navigation.isCurrentPanel('composer'))` block in `afterLeave` too.

Tell me if you already tried what I suggest and it didn't work?