Work with ProjectSignatures directly from the Version model.

Review Request #10321 — Created Nov. 15, 2018 and submitted — Latest diff uploaded

Django Evolution

This introduces a new SignatureField class, which wraps all
serialization/deserialization of project signatures, and updates the
Version model to use it. Callers will now see the new signature
objects, and won't need to worry about handling serialized data in any
form. This gives us flexibility later to change the serialization format
without breaking any callers.

All code loading in or saving signature data has been updated to work
with the new objects.

Unit tests passed.

Tested some basic evolution operations, which appeared to work. There's
still further development needed to get everything into place.