Geo: Selectively choose which projects to replicate in DR

This relates to using Geo for GitLab.com: https://gitlab.com/gitlab-org/gitlab-ee/issues/1884 Right now to use Geo DR, everything must be replicated. For many customers, they want to selectively choose which projects to replicate.

We probably want to build this for ourselves, since to test DR we probably want to incrementally add projects over time instead of turning on the fire hose.

Proposal:

1. Allow admins to select which groups to replicate
2. All project metadata still gets replicated because of PostgreSQL, but repos that have not been selected will be empty