Deprecate "Contact" column of Registrations table

[aarongable]

We announced that we are breaking the link between ACME accounts and email addresses. As part of that, we'll be deleting all contacts in the Registrations table. But today, that column is NOT NULL, and uses "[]" as the value for accounts that have no contacts associated with them.

We can overwrite all extant contact addresses with "[]", but it would be nicer to simply drop the column. And we'll need to drop the column eventually, either way.

This ticket tracks the work necessary to make Boulder treat that column as optional, so it can be dropped without causing any issues in the SA.