Redirect HTTP Git push from secondary to primary

As @jramsay wrote in https://gitlab.com/gitlab-org/gitlab-ee/issues/3834, and related to https://gitlab.com/gitlab-org/gitlab-ee/issues/623#note_43446269:

Asking users to manually update the push URL every time they clone a new repository is tedious. We should make it possible for people to do both fetch and push operations using the secondary by proxyingredirecting the push operations to the primary.

This makes easier for people to use geo because they only needed to grab the closest secondary when cloning and they're good to go.

Users are also mostly static relative to secondary nodes, unless they regularly travel large distances, and will always want to use the same secondary. We should allow users to save their preferred secondary.

Proposal

Start with HTTP and Git push only for simplicity.

Any properly authenticated push request should be proxied to the primary.

Links