Reflect organizer level data feeds, reference route guide standard

[howaskew]

As per the discussion on issue #313 we have updated this draft of the opportunity data specification to reflect the requirement to consistently share data about clubs or organisations that provide opportunities for Physical Activity.

[howaskew]

NOTES:

1. The changes to reflect organiser level feeds are non-breaking changes. They mainly consist of edits such as:

Before:
Events have organizers. An organizer might be a Person or an Organization.

After:
Physical Activities have organizers. An organizer might be a Person or an Organization.

This reflects the idea that a club providing activities can be considered an 'opportunity' even if the details of specific dated and timed events are not published, or not yet published. Some introductory text in the abstract explains this. The full implementation of OpenActive enables a seamless end-to-end discovery and booking of opportunities - and this is very much still the prime focus of the specification.

2. Earlier work in the Editors Draft to incorporate the Route Guide specification has been amended to point to the standalone specification and to indicate for example how a route guide could be attached to a 'guided walk' event.

3. The data model image has been updated to reflect events linked to route guides and organisers linked to activities.

4. To avoid confusion and simplify editing, we have used Organizers and Organization throughout (rather than mixing in Organisers and Organisations). This aligns with schema.org, the basis of the OpenActive specification.