Geo: add settings to allow disabling proxy to primary

Proposal

Currently there is a feature flag (geo_proxy_direct_to_primary) to disable "proxy to primary" on the Git access code. We should move that into a settings (which enabled proxying by default, to keep existing behaviour).

This should allow users with specific requirements to disable git proxy behavior.

The code change is relatively small, as we are going to replace a feature flag check for a settings check, see: geo_git_access.rb