Public mode scheme should be configurable

[David Coles]

At present, public mode URLs uses the same scheme as the main IVLE. Fine when we're only using HTTP, but causes problems when we wish to use HTTP for public mode and HTTPS for everything else.

At present, I'm just going to hardcode the public URL to 'http://' but this really should be configurable. Ideally we should replace the ivle.conf option of public_host (usually 'public.localhost') with public_base (default of 'http://public.localhost').