Notifications can leave a ghostly heading-notification behind

[gnprice]

This is an issue that doesn't exist in any released version; it was introduced with the spiffy new form of notification in #4842, and I discovered it later yesterday while preparing some refactors to that code after merge.

The repro recipe is:

• Have notifications active for several conversations in the same account; e.g., have another user send PMs in two different threads.
• Go read all those conversations at the same time, e.g. by going to the "Private messages" view in the web app.
• Expected: The notifications disappear.
• Actual: The notifications for the individual conversations disappear. But an empty shell of a header notification remains. It looks like this screenshot:
  
  (which was for a previous bug, fixed before merge, that once it was triggered had the same symptom.)

The root of the issue is that in the new removeNotification we're relying on an assumption that the loop looking for notifications to cancel found exactly one such notification. If it found more than that, the code won't realize if the summary notification should also be cancelled.