🚥 allow CONTENT_CDN host to be spec'ed

[stevejpurves]

fixing #342

This allows the hostname used in the CONTENT_CDN to be specified independent of PORT or HOST.

Note trying to use HOST for this purpose is problematic as the express development server attempts to bind to this and fails, we really just want to influence rewriting on urls to static assets, so we are adding CONTENT_CDN_HOST in a similar vein to how CONTENT_CDN_PORT is used.

A parallel change in mystmd is needed in order to add CONTENT_CDN_HOST based on HOST when starting the dev server (potentially only when --keep-host is used?)

[stevejpurves]

tested this by setting a name on /etc/hosts and then HOST=http://noplacelikehome myst start --keep-host with the output from make build-book copied into _build/templates folder