Create a default at-first-registration intervention date on all new studies

[biblicabeebli]

• add "First Registration Date" as an automatically created intervention date at study creation.
• do nothing to existing studies, it's not worth the problems with relative schedules that could occur.
  • Yes they can create one with the correct name that will hook in.
• when a participant registers, check if study has this "first registration date" intervention, and only if the internal first_register_user is unset - then add the current date as this intervention value - if there isn't one populated already.
  • if possible, set the date based on device time zone, maybe study timezone.
• Change First Registration Date in the participant table download to say "First System Registration" - also it will be a datetime instead of a date, so it should be fairly clear that this is not the "First Registration Date" ... auto-intervention-field.