Magic kludge to use gateway_txn_id as the globalcollect subscr_id

TODO:
* Bury that assumption in the payments frontend instead.

Change-Id: I3e06458537a289f49fbfcca1aa14c60211e3cf5b
Merge remote-tracking branch 'origin/master' into deployment

Change-Id: I052b22a431de5a87d57125151db73ab17f62ceeb