Allow scoped access to another users account

[Half-Shot]

Specifically I'd like a set of users (A) to be able to log into another set of users (B). The set of users may be all of the users on MAS, or a subset. Ideally this would be a scope that could be requested as part of the login flow.

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#1563