Small SEO/markup fixes: canonical URL form, og:logo, twitter meta attribute, http link #224

Problem

Four small issues in _layouts/default.html and _includes/follow-links.html:

1. The nav links to /about but the canonical URL says /about.html — two URL forms for every page.
2. og:logo is not a real Open Graph property.
3. Twitter card metas use property= where Twitter documents name=.
4. The Stack Overflow link is plain http://.

(OG images for About/Certificates are separate: #103, #106.)

Proposal

Pick one canonical URL form and use it in both nav and canonical/og:url; drop og:logo; switch twitter metas to name=; make the Stack Overflow link https://. Update affected specs.

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