Code review comment for lp://staging/~gary/launchpad/move-events-to-filters

A worry: target.addSubscription did result in a subscription that had a level of NOTHING. We never exposed that, to my knowledge, instead relying publicly on addBugSubscription, which always explicitly set a level of COMMENTS. Now, this branch creates subscriptions without filters, which means addBugSubscription has the same result, but addSubscription does not. I don't think it is an issue, but I'd like a sanity check from an expert (gmb? :-D )