CLC-2802 Prevent bad URL when re-creating deleted bCourses site

https://jira.media.berkeley.edu/jira/browse/CLC-2802

This fixes a cache-related issue I saw while testing bCourses course provisioning by CCN. When we create a new unpublished site (or when testing), it's not uncommon to decide to delete the trial site and start over, maybe with a different list of sections. When I did that, the final URL I was given didn't work. Instead, I had to find my new site by going to bCourses & searching for it.

Our bCourses data-refreshing and provisioning code mostly bypasses normal proxy caching so as not to trip over our own shoelaces while changing bCourses state. But in this particular case, I slipped up & made a provisioning-specific proxy cacheable. As a result, when I asked for the URL of my second (final) course site, I was given back the obsolete URL of the first (deleted) one.

If this is merged, I'd like it to also go into the QA branch for Sprint 30.