Fix front page critical CSS loading in IE.

The easy thing first - IE appears to 'stack' :before entries, so if a
stylesheet is loaded multiple times (e.g. due to critical CSS loading
on the front page), a font-size of 2.5em gets multipled. Switching to
rems fixes this.

The preload polyfill in IE8/9 loads the base stylesheet as if it were
after the layout stylesheet, which breaks the navigation display. The
polyfill has a new version which is somewhat simplified, updating the
existing <link> rather than creating a new one, so we upgrade to that
plus a PR to prevent browser hanging.

However, in IE8 this now incorrectly uses the print styles on screen,
meaning the navigation and footer disappear. So use an IE conditional
comment to just cut IE8 out entirely. This means we can also cut down
the polyfill somewhat.

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