Replace the Router used for API Requests

Context

Currently, the container registry uses https://github.com/gorilla/mux as its router. For a number of reasons, this router is not satisfactory. Including runtime performance, and developer friendliness.

A good recent summary of the "state of the art" on this subject: https://www.alexedwards.net/blog/which-go-router-should-i-use

Proposal

A preliminary analysis seems to narrow the field to https://github.com/go-chi/chi or https://github.com/julienschmidt/httprouter.

Ideally, we should strive to change one route at a time to the new router. This will help ensure that that new router performs appropriately on GitLab.com