Add infrastructure for recording the remote product for a project

At the moment we make no attempt to record the ID of a Product on a remote bug tracker when we link a Product to that bug tracker. This isn't usually a problem, since we import bug statuses by remote bug ID rather than remote project name / ID.

As start providing links to upstream bug-filing forms, however, we need to know the ID or name of the project on the remote bug tracker so that we can direct people to the right form.

Examples of this:

 * For Bugzilla, we need to know the product name for the remote Bugzilla (e.g. http://bugzilla.example.com/enter_bug.cgi?product=Fronobulator).
 * For SourceForge we need to know the product's GroupID and ATID.

For other bug trackers this may not be so important (for example in Trac the project name is part of the root URL, e.g. http://trac.example.com/myproject/newticket).

We will know this bug is fixed when:

 * The remote product of a Product is accessible as an attribute of that product
 * The remote product of a Product can be set using the Launchpad API