Refactor feed types

[nickevansuk]

We need some additional metadata to be included in Dataset Sites regarding feed data structure to allow an automated agent to determine which feeds are available, and the relationships between them.

See https://lists.w3.org/Archives/Public/public-schemaorg/2019Jan/0019.html for the question asked in the mailing list on this.

At the moment it’s hard to figure out which feed is which, and what combination of feeds are in use, based on the metadata.

Additionally, the data for which feeds are permissible is scattered across a number of disconnected locations:

• https://github.com/openactive/data-models/blob/master/versions/2.x/meta.json#L9 (which automatically populates https://github.com/openactive/OpenActive.DatasetSite.NET/blob/master/OpenActive.DatasetSite.NET/metadata/OpportunityTypes.cs)
• https://github.com/openactive/data-models/blob/master/versions/2.x/rpde/FeedItem.json#L39
• https://github.com/openactive/dataset-site-template-php/blob/master/src/FeedType.php
• https://github.com/openactive/dataset-site-template-ruby/blob/master/lib/openactive/dataset_site/feed_type.rb

Steps required:

1. Formalise approach to representing feed metadata in Dataset Sites via https://www.openactive.io/dataset-api-discovery/EditorsDraft/
2. Update the above to all reflect this, driven from a central configuration file (e.g. https://github.com/openactive/data-models/blob/master/versions/2.x/meta.json)