Fix signature not being updated when composer first opened without one

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 (closed)