Fix the sidebar on the /posts index page sometimes being blank. This
could happen when either the related tag calculation was too slow and
timed out, or when Reporbooru was unavailable and we couldn't fetch the
list of popular tags.
In the tag list would otherwise be blank, we fall back to frequent tags
(the most common tags on the current page of results).
Also change it so that if Reportbooru is unconfigured, we fail
gracefully by returning blank results instead of failing with an
exception. This is so we can still view the popular searches and missed
searches pages during testing (even though they'll be blank).
101e8a13b8