JobPost ⇔ Board needs a primary relationship

[jace]

We have a BoardJobPost model, but it represents a regular many-to-many relationship. There is no record of the board that the post was originally added to. This is necessary when we give board owners rights over job posts.

add_primary_relationship added in hasgeek/coaster#133 is an option, but since it needs a separate table, we could consider the other option discussed but set aside in that ticket: a primary boolean column with a unique constraint, with possible values of True or None.