Add constraint(s) for field_unique_id on nodes, media and taxonomy vocabs

[bseeger]

This is part of #383 and is dependent on #384, #385 and #386 being completed first.

We need a way to guarantee unique field_unique_id across types. It might be that they only need to be unique across entity_type, versus the entire system.

This doesn't necessarily have to be a constraint, but it needs to cover both cases where additions happen via the UI and migration.

Note: the unique id's are case insensitive, where, for example, uid:1243-BA07 and UID:1243-ba07 would be consider equivalent strings and not unique ids.

See also: https://docs.google.com/document/d/1WCu8bRtLELgGGoIBuyed0-pJB1GwgPuuGeLnMbCOaMM/edit#heading=h.l3aqgsytkn51

[bseeger]

Resolved with: jhu-idc/idc-isle-dc#194, but further refined by: jhu-idc/idc-isle-dc#198