root-relative hrefs in template links are not resolved properly

[cscheid]

This, in a website, will generate links to /cv.pdf and /blog.html regardless of the root directory of the website:

---
about:
  template: trestles
  links:
    - text: Blog
      url: "/blog.html"
    - text: CV
      url: "/cv.pdf"
---

[cscheid]

Punting to 1.4 because the fix involves navigationItem and we don't want the regression risk.

[cscheid]

@dragonstyle I'm thinking 1.5 for this one, and we make sure to give it a shot early on so there's time to shake out regressions.