Update header with appropriate OpenGraph metadata

[thelifeandtimes]

~lagrev-nocfep reports that title card image rendering, for open graph links and stuff on twitter and elsewhere, is not working as expected. We should update this to support the following:

• On the homepage, a custom image bespoke for urbit.org/ (@ahlmark)
• On the ecosystem page, a custom image bespoke for urbit.org/ecosystem (@ahlmark)
• On the overview, a custom image bespoke for urbit.org/overview (@ahlmark)
• On specific blog pages, the blog header image