(CE-1051) Properly cache when a SharedHelp article doesn't exist

When a SharedHelp article doesn't exist, the request returns a 404
and the SharedHttp call returns false, which wasn't being checked
before.
(CE-1051) Use wfSharedMemcKey to create SharedHelp cache key

The cache key was using $wgSharedDB to make a shared cache key, but
$wgSharedDB is different for each cluster (i.e. wikicities_c*), so we
were needing to cache once per cluster unnecessarily.
Merge pull request #5432 from Wikia/CE-1051

(CE-1051) Properly cache when a SharedHelp article doesn't exist