Per-user AWS auth

[paulsmith]

Right now we assume Soapbox is running in the same AWS account as the target deployments, but to be a pure SaaS/PaaS (to deploy into an existing customer's VPC, for example), we'll need to support a way to delegate auth to the user's AWS account.

A quick-and-dirty way would be to ask the user to supply a AWS credential key + secret pair, but AWS advises against this and recommends the use of Cognito, which appears to serve a similar function as OAuth.