update-sourceforge-remote-products should check that projects use SF for bug tracking

At the moment the update-sourceforge-remote-products script doesn't bother to check whether or not a given project uses SourceForge for bug tracking. If the project is linked to a SourceForge project and doesn't have a remote product, u-sf-r-p will simply set the remote product regardless of what the project uses for bug tracking.

This is a problem if:

1. The project uses a non-Launchpad, non-SourceForge bug tracker to track its bugs
2. That bug tracker supports multiple projects.

If both of these turn out to be true we could (most likely will) end up linking to bogus bug filing and search forms on the +choose-affected-product page.

What should happen: u-sf-r-p should check that the project it's setting the remote product for uses SourceForge to track bugs before setting the remote product based on the project's SourceForge project.