add `link` tag for W3C-standardised manifest (bug 1084548)

dependency: mozilla/fireplace#799

r? @ngokevin 

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note: the manifest must live at the same origin as the document, so I can't put the manifest in the `media` directory/the CDN.
supposedly, I can [override in the CSP](http://w3c.github.io/manifest/#h3_content-security-policy), but I'm unclear.