enable relativeUrls: true for all sites

[gumaerc]

As part of the efforts to consolidate the differences between the online and offline themes in mitodl/ocw-hugo-themes#1013, we should set relativeUrls: true in the site configs to tell Hugo to render relative URLs by default. This should only be done when the corresponding changes in ocw-studio and ocw-hugo-themes are ready to go as it will be disruptive if deployed before the changes are made to the way the URLs are rendered.

[pdpinch]

@gumaerc should this still be done?

[gumaerc]

@pdpinch No, as we decided not to pursue consolidating the online and offline themes: mitodl/ocw-hugo-themes#1013 (comment)