Sometime in the deep past, Zulip the GET /users/me/subscriptions
endpoint started returning subscribers. We noticed this and made it
optional via the include_subscribers parameter in
1af72a2745, however, we didn't notice
that they were being returned as emails rather than user IDs.
We migrated the core /register code paths to use subscriber IDs years
ago; this change completes that for the endpoints we forgot about.
The documentation allowed this error because we apparently had no
tests for this code path that used the actual API.
95606a7347