Migrate to OpenAPI 3.0

[preaction]

Now that the JSON::Validator plugin supports OpenAPI 3.0, we should finish the openapi-3 branch (or start migrating to OpenAPI v3 from scratch). OpenAPI v3.0 provides the following benefits:

• Better security definitions that allow for routes to request one authentication mechanism from a set of them.
  • It would be awesome if Yancy started using something like this to define auth in general (#40)
• Support for Links (which would improve the API definition for data relationships)
• Callbacks. These are not definitions for WebSockets, unfortunately, but still an interesting concept for making it easy to build microservices with just Yancy.

More information: https://swagger.io/blog/news/whats-new-in-openapi-3-0/

[preaction]

Instead, I'm removing OpenAPI from the editor entirely. If OpenAPI v3 support is needed, it will need to be done in Yancy::Plugin::OpenAPI