Open API v3 support

[shuttle1987]

Issues such as #111 require that we have OpenAPI v3 support.

The main blocker we had to this previously was a lack of support in connexion which we use for URL routing with API spec support. Now that this has landed upstream we should consider moving the API here to OpenAPI v3.

Would appreciate input here.

[aapeliv]

Sounds good to me. The codegen I'm using is straight from Swagger, so presumably it supports v3

[shuttle1987]

Just a note that we have to update the documentation on the API specification if we make this change to version 3. https://github.com/persephone-tools/persephone-web-API/blob/master/docs/APIspecification.rst