Switch to URL arguments rather than URL chunks for accept-invitation and opt-out page

Changes in BuddyPress 2.1 (https://buddypress.trac.wordpress.org/changeset/8658)
broke IA's old system for catching 'accept-invitation' and 'opt-out' URLs. We
avoid this problem by switching to URL arguments.

Backward compatibility is included for existing invitations.

Fixes #119