Ensure composer state is restored when loading draft replies and that
`referred_ids` is updated top include the messages the draft refers to.
Make the process for loading different context types more obvious by
doing the work in-place, especially for replies, and avoiding multiple
utility methods that also branch based on type.
Use standard RFC822 classes for managing In-Reply-To and References, and
merge id lists rather than simply concatenating them. Ensure both are
updated when adding additional replies to the email, and they are always
set on any composed email constructed.
Add unit tests for all of the above.
Defining an interface for the composer to access application objects
and services decouples it from Application.Controller, allowing it to be more
easily unit tested.
Replace use of Application.Client and Application.Controller in the
composer and add some basic unit tests.
