add REST documentation for activity module

Some thoughts.

* We should be careful relying on "try it out" since we've converted to using form parameters instead of body parameters; many of the endpoints (those that POST, anyway) won't actually be usable with "try it out" since they rely on form parameters that the Swagger UI cannot (currently) support. My guess is slightly less than half of the POST endpoints fall into this category; that's a significant number.

* The `{...}` convention that I've used for mapped objects works okay for documentation (as long as the user is aware of it), but I don't _think_ it will work so well for other uses of Swagger such as automatically generating client libraries.

There is another possible approach, though I'm not actually in favor of it. We could modify the API endpoints to conform to the constraints of Swagger. It probably wouldn't be a huge amount of work, and I suppose it would make the API a bit cleaner in general, but it seems perverted to change the interface because of limitations in the documentation tool.