Instead of deleting a namespace synchronously as before, we now mark the namespace for deletion, disable it, and rename it. A worker then comes along and deletes the namespace in the background. This results in a *significantly* better user experience, as the namespace deletion operation now "completes" in under a second, where before it could take 10s of minutes at the worse.
Fixes https://jira.coreos.com/browse/QUAY-838
Instead of disabling repo names with periods in them, we simply disallow calls to the API when they are GET requests, whose path ends in a dot, and that do not have a referrer from the frontend.
