The subdomain and host parts are already used within zulip_url, api_url,
this commit adds them as context properties themselves, so that they
can be used in email addresses and URLs without the URI scheme,
increasing uniformity.
The new context properties `display_host` and `display_subdomain` are
usable in the Help, API docs and Policies.
Occurrences of `*.zulipchat.com` have been updated to use them.
dab97304f6