Get licenses from embedded schema, skip bad modules in deserialize

[HebaruSan]

Problems

• Every time we add a new license, the previous version of the CKAN client is unable to read modules that use it. This is supposed to be handled by spec_version, but the user can bypass this protection by updating the registry with a new client and then running an old client, and they have actually done this (see CKAN.BadMetadataKraken: The license MPL-2.0 is invalid #3331 and CKAN.BadMetadataKraken: The license MPL-2.0 is invalid #3410 and a reply on the forum thread).
• We currently maintain the list of valid licenses in three places, which is annoying:
  • CKAN/Core/Types/License.cs
  • CKAN/CKAN.schema
  • NetKAN-Infra/netkan/netkan/mirrorer.py

Changes

• Now if any CkanModule in the available modules list throws an exception during deserialization, including but not limited to an unsupported license, we log a warning and omit that module from its AvailableModule collection. This allows the user to keep using the client, albeit without access to such modules.
  • This is done with a new JsonConverter that adds each version/module pair to a SortedDictionary one at a time and returns it containing whatever values didn't throw.
• Now CKAN.schema is an embedded resource in Core, and Licenses.cs retrieves its license data from there.
  • A CKANSchema class holds a schema object generated by extracting the embedded resource and then parsing it with NJsonSchema
  • Netkan's schema validator now uses CKANSchema.schema instead of its own embedded resource
  • A .definitions.redistributable_license enum is added to the schema to document which licenses allow us to redistribute.

[DasSkelett]

Nice deduplication