Improve JSON-LD Schema

[ldodds]

• Added definition of oa:ActivityOpportunity. This overlaps almost completely with event.
• Schedule needs more testing
• Some properties, e.g. schema:image can be a URL or an array. This isn't supported yet, but easy to add
• Place nesting via containedInPlace and `containsPlace1 is currently very permissive, need more constraints
• Add some support for custom JSON-LD contexts