If Oxford does not know discharge date when child joins, allow record submission and check Oxford for updates

[KateCourt]

• [x] Message for user that says not to submit record until after discharge
• [ ] Message should disappear once have discharge date from Oxford
• [x] When child joins, if discharge date not present, add null to firestore
• [x] Remove all references to supplement start date as no longer needed (from flutter)
• [x] Remove all references to recruited before or after discharge (from flutter)
• [x] When cloud function runs, if discharge date is null, check of Oxford has a value and update in firestore if there. If not there do not include child in push notification.
• [x] When add child save parent's email to the record

[KateCourt]

Cloud function:

• [x] Date saved already
• [x] Date not saved, oxford does not have
• [x] Date is saved, oxford does have, update firestore