Interpret pbs_birth_cohort flag during sync. Closes #3510.

This flag is not currently ever set by `Psc::SyncLoader`. This means
that eligibility events will always be sync'd using the PBS provider-
recruited PSC segment. In the future (#3512) we will need to determine
how to set this flag appropriately in `Psc::SyncLoader`. (For now we
are not supporting birth cohort syncing from Field.)
Merge pull request #63 from NUBIC/3510-psc_sync_pbs_elig

Interpret pbs_birth_cohort flag during PSC sync. Closes #3510.