Consent manager: `/users/@me/consents/:id`, `/users/:id/consents/:id`

Is your feature request related to a problem? Please describe.
Certain servers might want to provide access to third party services in ways that require consent.

Describe the solution you'd like
To manage consents to them, I am proposing a simple RESTful consent manager.
PUT /users/@me/consents/:id: Consents to the service #ID. If the ID refers to a non-existent service, silently ignore the error and return success.
DELETE /users/@me/consents/:id: Revokes the consent to the service #ID. If the ID refers to a non-existent service, silently ignore the error and return success.
QUERY /users/:id/consents/: Search consents of a given user. Requires MANAGE_USERS.
QUERY /users/@me/consents: Search own consents.
GET /users/:id/consents: Retrieve all consents of a given user. Requires MANAGE_USERS.
DELETE /users/:id/consents/:id: Forcibly revoke a user's consent to the service #ID. Requires OPERATOR.
DELETE /users/:id/consents: Forcibly revoke all of a user's consents. Requires OPERATOR.

No files, tests, or entry points are named. Start by locating the server's user and service API routes and the existing permission checks, then determine where consent data is represented. Done means the listed consent-management operations and their stated permission and missing-service behaviors are implemented and tested.