add REST documentation for activity module

>  Any cases where the Swagger UI cannot support Try it out because we're using FormParams should be switched to body params, but I think there would only be a few like this.

Well, there are a bit more than a few (slightly less than half of our POSTs in my estimate), but Brandon was equally insistent last week that we should use form parameters even if it broke the UI. I guess since he's not here any more :( I can undo all the work I did last week to convert to form parameters.