Local dev story for API

[mrosack]

I think we can refactor the API to support running 100% local without any real AWS resources. This will help with allowing other devs to work on the API without deploying to AWS, and also allow us to write some decent unit tests.

• Abstract out all direct AWS calls to allow unit testing
• Use https://github.com/99xt/serverless-dynamodb-local to run dynamodb locally
• Use https://github.com/mj1618/serverless-offline-sns to run sns locally
• Create mock S3 implementation that stores files to filesystem/in memory
• Create mock SES implementation that writes emails to filesystem/in memory