Show link to sanity test failure when displaying multiple

Start caching sanity test failures.

With all the fallout from the introduction of flavours, we have many more sanity test failure,
some with very long lists.

We never cached STF before, but now we do.

Since STF are neither ports nor files, but they relate only to commits, I created a new cache
classe in classes/cache-general.php

While here, classes/cache-port.php & classes/cache-file.php were slightly altered to use a
class constant, much like classes/cache-general.php

Code related to packages was removed from www/sanity_test_failures.php - I suspect
it was never used and was left over from a copy/paste situation.

I attempted to avoid file manipulation by using filter_var() in www/sanity_test_failures.php
when accepting incoming message_id values.

I see now that I am caching all results, not just successful reads from the DB. Could
someone fill up the cache via that? Yes, I'm sure they could.