Automate blue/green updates for the browser/API

[sjahl]

Still testing/in-progress, but I needed to push this somewhere to start triggering builds.

The way I intend for this to work going forward:

1. Blue and Green deployment definitions live over in https://github.com/broadinstitute/gnomad-deployments
2. PR to change the browser/API is approved and merged in this repo
3. API/Browser Docker build starts
4. On build success, update either blue or green kustomize overlays (whichever is currently not active)
5. push that change to gnomad-deployments main

After verifying that the changes look good, developer runs a blue/green swap cloudbuild action to redirect the prod traffic. Rollback can be done by re-running that job.