+ubuntupkg can create duplicate packaging links

The view does not verify that the distroseries + sourcepackage is not already used. The user is either making a mistake, or the user needs to correct the mistake before the link can be made. This code is embedded into the ProductSeriesView. We want this to be a LaunchpadFormView, and it probably should extend +addpackage view to ensure the rules are correct. There are two key differences that this +ubuntupkg does differently than +addpackage. It restricting the distroseries to the Ubuntu focus of development, and it records the change in the history.