Remove any N+1 issue in the generation of packages index

This was done by simply observing the logs and seeing what ActiveRecord was
loading.

The `get_packages` method was rewritten, to stop using direct SQL statements but
also to make use of eager loading.

The method that was looking up information if a package is in shipped-list has
been disabled (`in_shipped_list?`)since it is too slow (took ~ 8s).
Enable caching again!

Expire the cache when bz_bug is added to a package or the package itself is
edited.
Fix some bugs with duplicate entries in errata stuff
Merge branch 'errata_rpmdiff' of github.com:thescouser89/ett into errata_rpmdiff