2151: Add sorting on "created"

Missing db field, "created" on TeamMember.  Can be inferred from the Activity
model.
2151: Add migrations
2151: Actually add the created field
2151: Add TeamMember.created to admin
2151: welp, order_by() isn't in place
Merge branch 'i-2151' into production
Merge branch 'production' of github.com:pculture/unisubs into production