Set header logo URL via configuration

[arbrandes]

Description

At the time of writing it's not possible to set the header logo URL via a single configuration variable, which used to be possible via the LOGO_URL variable. For the sake of easing migration, this functionality should be restored, perhaps via a simple logoUrl site.config setting.

[brian-smith-tcril]

For now, just focus on handling one LOGO_URL. We can worry about handling different URLs for different apps later.