This new `DISCOURSE_MAXMIND_BACKUP_PATH` can be used a secondary location
for maxmind db. That way a build machine, for example can cache it on the
host and reuse between builds.
Also per 5bfeef77 added proper error raising for download fails from
dedicated rake task
This also moves "refresh_maxmind_db_during_precompile_days" to a global
setting, it did not make sense in a site setting