DM: API for joint device list

[matrixbot]

This issue was originally created by @Johennes at matrix-org/matrix-authentication-service#1549.

For matrix-org/matrix-authentication-service#1535 we require an API that combines the current APIs for the three different lists. Technically we could combine the three APIs on the client-side but since they're paginated this easily gets unwieldy enough that providing a dedicated API is probably lower effort (and cleaner to begin with).

[matrixbot]

This comment was originally posted by @sandhose at matrix-org/matrix-authentication-service#1549 (comment).

We've been doing this for apps for quite some time, it's just browser sessions that are still separated; we'll have to reevaluate what we do with those later