Add a "opaque_id" column to submissions.

[veluca93]

This gives a stable ID that can be used to refer to a specific submission.

Quick SQL snippet to update an existing database:

begin;
alter table submissions add column opaque_id BIGINT UNIQUE;
update submissions set opaque_id = id where opaque_id IS NULL;
alter table submissions alter column opaque_id SET NOT NULL;
commit;