Settle on environments

[afeld]

We have a few notions of "environments" in our Azure environment right now:

• App Service slots
• Development vs. Production subscriptions
• Resource groups — we just have one, but could have multiple

We should settle on which we intend to use (at least for now), and clean up the rest. @cal-itp/cdt-devsecops Would love your guidance here!

Acceptance Criteria

• Documentation/infrastructure/pipelines updated to reflect our decision

Additional context

• https://docs.microsoft.com/en-us/azure/architecture/solution-ideas/articles/dev-test-iaas
• https://docs.microsoft.com/en-us/azure/app-service/deploy-best-practices#use-deployment-slots

[afeld]

Per @paramesh5235, the intention is that slots are used in the Production subscription / resource group for previewing changes to the app. The Development subscription / resource group will remain mostly empty, and we can create resources in there to play around with as needed.

[afeld]

Closed by #691.