This refactors get_members_backend to return user data of a single
user in the form of a dictionary (earlier being a list with a single
dictionary).
This also refactors it to return the data with an appropriate key
(inside a dictionary), "user" or "members", according to the type of
data being returned.
Tweaked by tabbott to use somewhat less opaque code and simple OpenAPI
descriptions.
This adds get_single_user to python_examples.py in zerver/openapi.
This also adds get-single-user.md to templates/zerver/api and adds
get-single-user to rest-endpoints.md (templates/zerver/help/include).