Soft logout: Support SSO auth

Make sure we can log in back after a soft logout on a homeserver running SSO auth.
Information to test against a SSO auth homeserver is pinned in the #softlogout-dev room.

Riot-Web PR: matrix-org/matrix-react-sdk#3197

- [x] Check soft logout display on SSO
- [x] If the hydrated user ID doesn't match the session's user ID, overwrite (done in https://github.com/matrix-org/matrix-ios-kit/pull/584/commits/8a296097638639401e37a3e08b8b02674201ecac)
- [x] Pass the `device_id` in the fallback URL we use for SSO
- [ ] Final test when `device_id` is implemented on the fallback page (blocked by https://github.com/matrix-org/synapse/issues/5755).