Expose record's url in API responses

When the API is created with a `baseUrl` and `apiBaseUrl` option
it can use that to generate links within the API output. This also
works on sub-documents, so the embedded memberships will have url
properties as well.
Make the filter module act as a transform function

Mongoose allows a `transform` option to be set which changes the return
value of `toJSON`. This change makes the filter module's method
signature match the transform function signature so that it can be used
in isolation.