add REST documentation for discussions module

requires common models in PR https://github.com/oaeproject/Hilary/pull/924
As a note, since the Swagger UI doesn't support arrays for form parameters, I did make all form parameters primitive types. If we want to make them all arrays, I can handle that in the follow up PR for consistency. (I'd also change the description to be something like "... ; can be single string or array of strings" instead of "... ; may also be array"