Support Disabling Geo Replication by Resource in UI

Problem to solve

As @fzimmer stated in #213885 (comment 327896598):

This is a more general point where in the SSF it may be desirable to default to on but to be able to disable replication on a case-by-case reason. If you don't need packages replicated for geo-replication, we should allow users to disable it.

As we continue to support more resources with the self-service framework, unnecessary replication will become more of a problem for users who only care about replication for particular resources.

Intended users

Sidney (Systems Administrator)

Further details

Proposal

Allow admins to enable/disable replication per Replicator per secondary via the web UI.
Migrate feature flags to the new settings so anyone who has disabled anything by feature flag is not adversely impacted
Add any documentation and code needed to support developers adding new resources via the self-service framework
Until secondary mimicry is implemented, ensure there is a warning that secondary functionality may break due to missing files

Permissions and Security

Documentation

Availability & Testing

What does success look like, and how can we measure that?

Users have an easy way to disable and enable replication for specific resources. Engineers are easily able to add this functionality for new resources using the self-service framework.