Automate deployments from GitHub to Elastic Beanstalk

[victorlin]

I did this for serratus-api using AWS CodePipeline+CodeBuild, but it's a bit nontrivial here since the project relies on secret values inside an untracked appsettings.json. So far we have gotten around this with deployments from local machines.

AWS Secrets Manager seems to be the way to go. I've looked into it a little bit and looks like we can:

1. get them during deployment-time as environment variables (source), or
2. reference them in the code directly

I'm thinking the 2nd option is easier, we'll just have to update the configuration builder.