Document API with OpenAPI spec (API 1.1) (#420)

* Add initial OpenAPI spec

* Add endpoint categories

* Update info and add version

* Define API endpoint

* Finish Get Member Info-endpoint

* Finish Get Member Balance-endpoint

* Correct descriptions and add missing descriptions

* Update schemas to include member_id and username

* Finish Get Member ID-endpoint

* Bump version to fit version in #421

* Rename operation IDs to URL names

* Finish Get Member Active-endpoint

* Finish Get Member Sales-endpoint

* Rename product_price schema

* Finish Get Payment QR-Endpoint

* Add NamedProducts-Endpoint

* Add response for missing param, refactor res-value

* Rename stregdollar to stregoere

This was done to make it more clear, what the value actually is

* Refactor named_products schemas

* Add Active Products-endpoint

* Draft category mapping-Endpoint

* Finish Dump Category Mapping-endpoint

* Progress related to documenting Sale-API

* Expand Sale-API response

* Add Ballmer-peak measures

* Fix invalid response format

* Fix wrong keywords

* Reduce 200-example

* Make api/member-endpoint valid

* Make multiple api/member/*-endpoints valid

* fixup! Make multiple api/member/*-endpoints valid

* Make endpoints requiring room_id valid

* Make api/sale-endpoint valid

* Make 'api/member/payment/qr'-endpoint valid

* Remove old individual responses

* Make '/api/member/get_id'-endpoint valid

* Change description to be more intuitive

* Add request body to 'api/sale'-endpoint

* Made request-example coherent with response-example

* Add description to 'api/sale'-endpoint

* Add local endpoint as server

* Rename file to stregsystem.yaml

* Rename parameters to unique IDs

* Convert examples as schemas into examples

* Rename operationIds to fit changes in #421