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.

[sjahl]

Just noting what the ship plan here looks like to get the automation pipeline started:

• Merge this PR
• Create enabled cloud build triggers: https://github.com/broadinstitute/gnomad-terraform/pull/161 sets up the scaffold, but just needs the enabled/disabled bits flipped
• Create a blue or green deployment in Argo (pick whichever color is not active)
  • Example application manifests: https://github.com/broadinstitute/tgg-sre/pull/85
  • Recommend that these start as manual Argo syncs for better control during the rollout, then switch to automated sync policies once things are in place
• Create the new prod bluegreen deflector service: see https://github.com/broadinstitute/tgg-sre/pull/85 for an example
• Point the prod ingress at the new bluegreen deflector
• Replace the other blue/green deployment once it is no longer active prod

Once these steps are complete, the desired end state is that pushing to the main branch of gnomad-browser triggers a docker build, and automatically rolls out the inactive blue/green with the new image. Then promoting that can be done with the manual blue/green flip cloudbuild job.