Cannot trust distroseries for report on campaign subscriptions

Currently we create subscriptions for campaign links as soon as possible from the "Thank-you page", but as a result, don't know the distroseries (we do grab it from the useragent, but otherwise use the default of the latest supported series). As a result, we cannot trust the distroseries for campaign subscriptions.

We could instead:
 * Make Subscription.distroarchseries nullable
 * Don't guess it for campaign subscriptions, but simply leave it null if we don't know (on the thank you page)
 * If the payment notification processor comes to create a LP subscription for a distro-arch-series-less subscription, it creates it for the latest supported (default) distroseries, but leaves the Subscription.das field as None
 * Then in initialize_purchase, if we find a Completed subcsription for the requested app, we double check to see if the Subscription alreay has a das, if it doesn't, at that point we can set it
 * We could also update the deb_line at that point too [1]
 * reporting for each app will also have to be able to cope with distroseries-less Subscriptions

[1] Could we instead stop storing the deb_line, instead simply store the token, and then make deb_line a property which would automatically display the correct thing (hrm, except when there is no distroseries for the subscription).