Geo: Spike skeleton of Repository Strategy for Snippets

Lay a foundation for replicating Repositories so it's clear where code should be, so we can parallelize the rest of the Epic work.

On a call with @vsizov about !38688 (merged), we settled on some rough big ideas:

• We need a ReplicatoryStrategy and a ModelStrategy for Blobs. These modules are naturally highly coupled. Maybe let's put both modules in one file. Same for Repositories.
• Don't try too hard to hide where/how event publishing is triggered, because every developer adding a Replicator class should consider "Do I need to trigger an event somewhere specific to this Model"?
• Let's get the code organization done in 13.3 without necessarily making any replication work.
• Consider breaking out some small initial refactor MRs.
• It should have no effect with the feature flag off
• It should not blow up with the feature flag on
• After this, the Epic will be like a checklist (in some cases, just verifying that something works)