Improve performance of PM Status checks

As I was restoring @sime's work in the MR, I realized something...we probably don't want to cache the actual results. That could be really problematic if you've cached a state of "no results", but an error condition is being created in the background.

What we really want to do here is run the status checks less often, I think. So what if we simply cached a flag that said "hey, we ran the status checks at such-and-such a time", and give it a 5-minute TTL? And when it expires from the cache, we run the status checks again.

And, just to cover our asses, we also run the status checks in \Drupal\project_browser\Controller\InstallerController::begin() (clearing the cached last-run-time first).