Add platform filter to bugsahoy interface

[staktrace]

For some bugs you need e.g. a B2G device or an Android device or whatever, it would be nice to be able to exclude such bugs. I'm not sure if an extra whiteboard tag would be the best way to indicate this or not since the platform fields often reflect the platform the bug was filed on or found on and not necessarily all the platforms the fix could be done on.

[jdm]

I once talked with rik about adding a [needs device] annotation (or the inverse), but the problem of figuring out an appropriate default and getting people to use it was difficult.