Restore working zulip implementation from before rebase
The merge conflict resolution during rebase incorrectly removed:
- user_id and delivery_email optional parameters from User.create
- get_by_id function from Users module
- client accessor function from Bot_storage module
This commit restores the working version from the original branch (324a64b)
that was in place before the rebase started.
The core zulip library now builds successfully.