If you added a Feed where a Show exists but it had no episodes
entries, the stack would end up in a populated state, but the
HomeView would be blank without widgets.
This changes it so the stack state depends upon the episodes
table being populated instead of the show. The downside
is that if your only feed is one without episodes you can
no longer navigate and interact with it.
This is never used anywhere else apart from the testsuite. Instead
of ignoring etags we should instead not save them if the feed does
not return 200 or 304. See #64.
