Consent Signature Image (REST API, DynamoDB object model, Send Mail)

Server-side implementation of Consent Signature Image. Includes updating the object models used by the REST API and by DynamoDB and updating the send mail service to write the signature as an embedded Base64 image.

Tested this with play compile and play testOnly for the tests I changed. Still need to run play test and play run to test the changes end-to-end.

NOTE: This isn't the final CR yet. This is just a preliminary review to make sure the overall design looks okay. We'll still need to rip out / fix up the email stuff and fix up the check for giving consent twice.
Some studies may collect a signature, but some may not. It's okay to let the client validate this until we're sure this 100% required for all consents (IMO).