Add machine-readable API description

[jtojnar]

This patch adds the description of the RESTful API in OpenAPI 3.0 format. Previously, the API was manually documented on the wiki without any guarantee that the description matches reality. Formal description will allow us to check for compliance automatically, as well as use other cool tools like Swagger UI.

For now, you can view the API on https://app.swaggerhub.com/apis/jtojnar/selfoss/2.18-SNAPSHOT

Closes: #993

cc @niol

[niol]

From a quick check:

• GET /logout seems to be missing
• GET /update seems to be missing
• GET and POST /opml seem to be missing, GET /opmlexport also
• GET /items/sync is missing
• the documented calls are looking consistent with the current implementation

[jtojnar]

I only documented the “stable” API calls listed on the wiki. I think /items/sync is internal and only used by web client at the moment, /logout and /update should be documented though.