users: Add get_user_data function.
users: Add get_user_data function.

This commit adds get_user_data which is called by
get_members_backend to compute the client_gravatar
value and then return the data of a single user or
all accessible users.

This function will also be used by get_user_by_email
in further commtis.

zulip/zulip,kou/zulip,rht/zulip,andersk/zulip,zulip/zulip,kou/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,kou/zulip,rht/zulip,kou/zulip