it was being used in ways that implied it should be NOT NULL
even though it was possibly null. we used to get this data
from the satellite db's added_at column as seen in 30369b02,
so backfill it using that data where joined_at is NULL, and
then alter the table to constrain the column to be NOT NULL.
Fixes#3866.
Change-Id: If2d856189209740d985f71dada7b93525e625ef3