subscription does not validate uniqueness of email per list
as discussed, otherwise we get the SQL constraints thrown at us:
PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "index_subscriptions_on_email_and_list_id"
DETAIL: Key (email, list_id)=(foo@example.com, 1) already exists.
: INSERT INTO "subscriptions" ("list_id", "email", "fingerprint", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"