Added FirstPromoter referral information to Stripe checkout session when available

closes https://github.com/TryGhost/Team/issues/1038

- FirstPromoter integration needs the referral tracking id information as part of checkout session object to correctly attribute member referrals
- Adds `fp_tid` metadata key for FirstPromoter to correctly attribute paid signups

ghost/portal/src/utils/api.js

@@ -152,6 +152,7 @@ function setupGhostApi({siteUrl = window.location.origin}) {
                     metadata: {
                         name,
                         requestSrc: 'portal',
+                        fp_tid: (window.FPROM || window.$FPROM)?.data?.tid,
                         ...metadata
                     },
                     successUrl,