upd: terminology in database

upd: terminology in database

The userGroup enum is updated to remove all assurance terms, and also
remove the (unused) arbitrator role entirely.

Since PostgreSQL offers no way to rename or drop enum values, we create
a new enum, migrate the table to it and then drop the old enum.

Change-Id: I200c2b0463ded9d75b2e963d5a02bfc25326b357