feat(api): API specification v2.19.0

feat(edu): added new `POST /eduResources/{resource}/createLtiLink` endpoint to create LTI links from Flat for Education resources.
feat(edu): maximum graded has been increased from 100 to 10000.
feat(score): new nullable properties in `PUT /scores/{score}` to unset properties (`subtitle`, `composer`, `lyricist`, `arranger`, `description`, `creationType`, `license`, `licenseText`).
fix(score): wrong user object in `ScoreSummary`, returned object has more properties than the specification (`UserPublic`).
fix(spec): various properties marked as required as they will always be returned.

Author: gierschv