Convert the Swagger specification of the REST API to OpenAPI 3

[osma]

Description of the enhancement

Convert the Swagger 2.0 specification of the REST API to OpenAPI 3.

This could be done using the same approach that was used for the Annif API specification: NatLibFi/Annif#649 i.e. use the Swagger Editor for an initial conversion, then manually fixing things that broke.

Who are the users that would benefit from the enhancement and how?

Any users of the Skosmos REST API

What new functionalities would the enhancement make possible?

This is just basic housekeeping, staying up to date with the evolution of OpenAPI specifications and tools.

[joelit]

While we're at it, we could look at autogenerating REST documentation as mentioned in #1406
From the top of my head, something like these could be a nice tool/starting point:

• Restler
• Swagger-php