FEATURE: add backup directory for mmdb files

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

config/discourse_defaults.conf

@@ -208,3 +208,10 @@ max_old_rebakes_per_15_minutes = 300
 # maximum number of log messages in /logs
 max_logster_logs = 1000
 
+# during precompile update maxmind database if older than N days
+# set to 0 to disable
+refresh_maxmind_db_during_precompile_days = 2
+
+# backup path containing maxmind db files
+maxmind_backup_path =
+