feat(api): API specification v2.6

* feat(collections): Add new Collections API endpoints
  * `POST /collections`: Create new collection
  * `GET /collections`: List collections
  * `GET /collections/{collection}`: Get collection details
  * `PUT /collections/{collection}`: Update collection details
  * `DELETE /collections/{collection}`: Delete collection
  * `POST /collections/{collection}/untrash`: Untrash collection
  * `GET /collections/{collection}/scores`: List scores contained in a collection
  * `PUT /collections/{collection}/scores/{score}`: Add a score to a collection
  * `DELETE /collections/{collection}/scores/{score}`: Remove a score from a collection
* feat(collections): Add new OAuth2 scopes for new features:
  * `collections.readonly`: Allow read-only access to a user's collections.
  * `collections.add_scores`: Allow to add scores to a user's collections.
  * `collections`: Full, permissive scope to access all of a user's collections.
* feat(score): Added new method to untrash a score (`POST /v2/scores/{score}/untrash`)
* feat(score): `DELETE /v2/scores/{score}` can now be used without admin rights. This new behavior will unshare the score from the current account.
* feat(score): `POST /scores/{score}/fork` now accepts a collection identifier to copy a score to a specific collection.
* feat(comments): Comments can now be filtered by type with the new query string `type` (`document` or `inline`).
* update(openapi): Some schema definitions have been renamed, they are now used for Scores and Collections
  * `ScoreRights` -> `ResourceRights`
  * `ScoreCollaborator` -> `ResourceCollaborator`
  * `ScoreCollaboratorCreation` -> `ResourceCollaboratorCreation`
  * existing _score sharing key_ -> `ResourceSharingKey`
* **DEPRECATED**: `GET /scores/{score}/revisions/{revision}/{format}` no longer support part indexes for single/set of parts exports, but our own part UUIDs.
* **DEPRECATED** on 2019-01-01: `GET /users/{user}/scores` will no longer list private and shared scores, but only public scores of a Flat account.