Country registration check

xF2 Add-on Country registration check 1.7

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Added error handling code if ipstack.com returns invalid json.
Added additional code to prevent null condition server error logs.
Fixed issue where ipstack.com is over limit and variable was returned as null.
Added 'Allowed country codes' to options page.
Added table to prevent repeated ipstack.com lookups.
Added Country registration check page to see which IPs have been blocked.