xF2 Add-on Signup abuse detection and blocking 1.15.6

• Require XF2.1+, drop XF2.0 support
• Harden Content Link Checker to hopefully not cause regex errors with some versions of php
• Support using a proxy service for port scans (bundled script/port_scanner.php provided. Host port_scanner.php on a webserver somewhere, and then enter the URL into the relevant add-on option)

• Fix that a user could be reported as multi-accounting themselves (ie token being upgraded/replaced)

• Add work-around for possible race conditions & duplicate key exceptions

• Fix error bumping a multi-account report