Give subscription a proper primary key

Otto Visserrequested to merge
LiamClark/queue-204-subscription-should-have-proper-primary-key into 199-database-migrations

The User (person) table had used the string value of the endpoint as a join column. A new id field has been added to the subscription table including a foreign key column into the person table.

The existing relation is preserved before the old join column is deleted.

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