add REST documentation for content module

However, there were some problems with how the `FormParams` were being used here:

- The form params were declaring `manager` and `viewer` instead of the correct `managers` and `viewers`
- Some parameters should have been optional (description, managers, viewers, etc.)
- The `File` type doesn't seem to be working (same for the body approach). Swagger UI doesn't give a file input field, whilst http://petstore.swagger.wordnik.com/#!/pet/uploadFile does, so we should investigate why that's the case