Allow Cross domain tracking with Google Analytics

Allow Google Analytics to share tracking data between PracticeHub and another domain. If a patient makes an online booking in an iFrame or link from another domain the referral data is lost in Google Analytics. By enabling cross domain tracking a user session can be tracked from a clinic's website to PracticeHub allowing more comprehensive analytics. i.e. Which Google ad campaigns led to more bookings. https://developers.google.com/analytics/devguides/collection/gtagjs/cross-domain