Improve external API

[russss]

Currently slotmachine accepts a fairly opaque list of dictionaries from the main website:

                export = {
                    "id": proposal.id,
                    "duration": proposal.scheduled_duration,
                    "speakers": [proposal.user.id],
                    "title": proposal.title,
                    "valid_venues": [v.id for v in proposal.get_allowed_venues()],
                    "preferred_venues": preferred_venues,  # This supports a list, but we only want one for now
                    "time_ranges": [
                        {"start": str(p.start), "end": str(p.end)}
                        for p in proposal.get_allowed_time_periods_with_default()
                    ],
                    "preferred_time_ranges": [
                        {"start": str(p.start), "end": str(p.end)}
                        for p in proposal.get_preferred_time_periods_with_default()
                    ],
                    "spacing_slots": spacing_slots,
                }

We should replace this with a class in the Slotmachine package which clearly defines the types of these fields, and documents them usefully.

I don't know if we can do something better with spacing_slots. We could probably move the concept of spacing outside of slotmachine entirely, and just feed it the total duration including changeover times, but I'm not sure if this might be useful for debugging.