Add one constituent identifier mapping

For GiveCampus constituent data and imports.

Open Online Giving → Reporting Settings → Constituent Identifiers. In Add Individual Identifier, enter Constituent Email and Constituent ID, then select Add Identifier. Verify it in Current Identifiers. This records an email-to-external-ID reporting mapping; it is not the same as creating a full imported constituent record. Earlier gifts require the supported scoped backfill if desired.