Utilize legacy_endpoint_id column (bug 1154918)

This column was created in migration 11 but isn't used because it wasn't
added to the model definition. Attempting to store data here ended up
putting it into 'extra'.

Change-Id: I02680b5213f09fe3cddcf5365104554d3e6d6b8a
Merge "Utilize legacy_endpoint_id column (bug 1154918)"