add REST documentation for activity module

I don't think we should be creating generic `Actor`, `Object` and `Target` models that contain a mixture of all of the possible values each of the different types might contain. It makes it unclear as to what will be returned when and makes it hard to maintain.

Instead, my preference would be to create `ActivityUser`, `ActivityGroup`, `ActivityContent`, `ActivityDiscussion`, etc. models that can then be enumerated next to `actor`, `target` and `object` in the Activity model.