The ComposerPageState JS object assumed that if no signature was present
when first loaded, that none ever would be. This broke changing the
signature when the composer was opened for an account without one, and
the from account was changed to an account with a sig.

Instead of including the signature as part of the loaded body, always
include just a skeleton signature DIV and ensure the signature is loaded
dynamically after the body has been loaded. Update code and tests to
match this assumption, and add a unit test for updating the sig.

Fixes #309