Add support for iotschema.org schemas

[benfrancis]

There's some interest in consolidating the capability schemas at webthings.io/schemas with those at iotschema.org so that multiple projects (e.g. WebThings, ThingWeb, openHAB and HomeAssistant) can eventually share a common standard ontology.

I think consolidating these schemas, and potentially migrating to iotschema.org over time, needs to be a multi-step process:

1. Add support to WebThings Gateway for iotschema.org schemas of capabilities/properties/actions/events for which we already have a user interface design
2. Propose iotschema.org schemas for the capabilities we currently implement in WebThings which don't have an equivalent at iotschema.org
3. Optionally, design and implement user interfaces for iotschema.org capabilities for which we don't currently have an equivalent in WebThings
4. If we're happy that the set of schemas at iotschema.org satisfy our needs, work towards deprecating support for webthings.io/schemas over time

[benfrancis]

See also: Evaluate potential consolidation with iotschema.org in the schemas repo.