Add kustomization for ERB deployments

[sjahl]

This adds a base kustomization file for the exome results browsers, and reduces the deployment manifests down to their lowest common denominator, so that we can easily extend them for multiple environments.

An example of how you might further extend the base kustomization can be found here: https://github.com/broadinstitute/gnomad-deployments/tree/main/exome-results-browsers

[rileyhgrant]

lgtm!

Thanks for taking the time to go over these changes, and more broadly about kustomize and other tools after standup today :), it was very helpful for my understanding.