Feature request #1729: Think about allowing non-unique product references in repo model

Right now, we're only allowing one repo per product.

If you try to add a second repo that also belongs to the same product you get a uniqueness violation.

I suspect that the uniqueness check doesn't actually buy us anything special here, and should probably be dropped.

Case in point: the Fusion project is a bunch of related repositories, but I don't want to clutter up the issue tracker with separate products for each repo, so I've created a single "Fusion" product and want to do everything within that product.

Will leave this ticket up for a bit while I think about it, but I think it's likely that I'll end up going ahead with it.