Add siteAdmin flag to db.
Add siteAdmin flag to db.

This lays the groundwork for incoming changes, but is kept separate
because even though the way the new field is used and included into the
model, the underlying field in the db should remain the same.

Issue #263: Admin users and User Administration screen

OpenTechStrategies/smoke-alarm-portal,OpenTechStrategies/smoke-alarm-portal,OpenTechStrategies/smoke-alarm-portal,OpenTechStrategies/smoke-alarm-portal