frontend: Refresh webook no longer uses update queue

Created by: ryanslade

The refresh endpoint is intended to be used as a way to manually refresh a repo on demand, as documented here.

This change avoids going via our existing repo-updater queue so that manual refreshes continue to work even when disableAutoGitUpdates is set.

Fixes: https://github.com/sourcegraph/customer/issues/109 Fixes: https://github.com/sourcegraph/customer/issues/371