This is a redo of https://github.com/matrix-org/synapse/pull/5897 but with `id_access_token` accepted. Implements [MSC2134](https://github.com/matrix-org/matrix-doc/pull/2134) plus Identity Service v2 authentication ala [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140). Identity lookup-related functions were also moved from `RoomMemberHandler` to `IdentityHandler`.