Add view_on_site method in course admin (#1411)

[ssciolla]

This PR aims to resolve #1411.

[jonespm]

I was testing this PR and it looks like this does work in production. This might just be a a misconfiguration on localhost with the sites table.

It looks like this fix you have here also works and goes directly to the link but whatever Django is doing to redirect is also working if the sites table is defined correctly. I think either of these is probably fine since this is a simple change.

[ssciolla]

Ah, you are correct, @jonespm. If I change it to my ngrok URL, or add my URL to ALLOWED_HOSTS, then it works as expected. Is my change here worth adding? Not sure...

[jonespm]

It's a small change, I'm fine merging it as it should work in all cases.

Otherwise we'd have to document to make sure the sites table is correctly configured. It looks like it should automatically configure it if the DOMAIN is set? I don't actually see DOMAIN in any of the sample configuration files though and not sure if I ever set it?

If we only ever set one site I'm not sure if we need the sites framework? Or maybe it would be useful if/when we put the LTI config in the database. Though CAE isn't using Sites.

[ssciolla]

DOMAIN gets populated from ALLOWED_HOSTS in env.hjson. I think we may need the sites framework, but not 100 percent positive.