[bug 943370] Add product-specific KB notifications.

This was kinda tricky but kinda easy at the same time. I have mixed feelings.

This includes:

* UI changes to the KB and L10n dashboards to make it more obvious what product you are looking at if any.
* New product filter for the notification events. I reused our existing events and added the product filter to them. I had to do some funky stuff for backwards compatibility so that people already subscribed are treated as if they are subscribed to all products (which they are).
* Tweaked the watch and unwatch views to handle an optional product.
* UI changes to the KB and L10n dashboards so that the notification options only apply to the current product if one is selected. Otherwise, they apply to all products. I added a note about that in the popup thingy.
* New tests for all of this.