Limits hook extension

[JDGrimes]

Previously: https://github.com/WordPoints/hooks-api/issues/5

Some good info in that ticket, including some (old) code.

I think that maybe we need this for the badges extension, though my brain isn't telling me why right now. Anyway, if we don't, we might want to drop this into a module instead.

[JDGrimes]

I don't think that this is vital to the badges component. Probably I was thinking about rates/quotas rather than limits, and was confused.

Of course, limits would be used by the badges component in the same way that they'd be used by the points component, to limit the number of times that a badge is awarded. A very useful feature, but perhaps not vital. Though not having them would make it awkward to create badges of the form "leaving your first comment". So I think it is safe to say that limits are integral to at least a certain class of badges, and a common class at that. So this does really seem like a basic badges feature.

[JDGrimes]

I also happened to think of the scenario where you want to create a badge that awards a user for commenting on 5 posts. Note that this is different than just leaving 5 comments on posts; comments must be left on 5 different posts. Thus, it doesn't seem possible to construct the badge solely with quotas and conditions. It made me think that limits would possibly have to come in here, so that we limited one count per post. But that doesn't actually work, because limits only control the number of times that the badge is awarded. So this would actually have to be a feature of the rate/quota API (#471).