R1:a2b2c391a1f1

Distinguish between "Assigned" and "Effective" identity PHIDs more clearly and consistently

Summary:
Ref T13444. You can currently explicitly unassign an identity (useful if the matching algorithm is misfiring). However, this populates the "currentEffectiveUserPHID" with the "unassigned()" token, which mostly makes things more difficult.

When an identity is explicitly unassigned, convert that into an explicit `null` in the effective user PHID.

Then, realign "assigned" / "effective" language a bit.