Tag all resources in CDK stack and force Enum of environments

[symroe]

There's a couple of useful things we're doing in CDK applications in other DC products, e.g:

https://github.com/DemocracyClub/UK-Polling-Stations/blob/master/cdk/app.py

1. We tag all resources
2. We force an enum of the DC_ENVIRONMENT

Both of these would be useful here too, and form a sensible starting point for the work of deploying to >1 AWS account.

[symroe]

The value of dc-product should be ynr

[mattpep]

I address this in PR #2544