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.
I would rather allow an object like this to safely collect what input has been sent from the client, and then validate the object, returning errors for everything that is wrong in one message. Just for the usability of the API, as people try and integrate with it, it is nice to get everything back that is wrong at once, rather than an IEE for a single problem, then another, etc. I don't know if the pattern was followed here, but the consent service can validate the object before calling any DAOs.