See problem statement in #2950

If I could set a template as backed by a git ref, change management is dead simple. Iteration shouldn't suffer as template update is replaced by git push.

CI automation (e.g pushing a Docker image with each template) also becomes simpler.

We should do the "Open in Coder" button alongside this as well. Keep in mind that this button should support specifying a path within the repository, as multiple templates can coexist in one repo.