Enable cache-busting URLs for CSS and JS

This will allow us to increase the cache times in the browser for our
CSS, which almost never changes.

Enables a new value to be used in templates, {{gitrev}}, which can be
used to bust pretty much any URL. We could do this for all the images
in the templates as well, but since most of them almost never change,
we'll just enable it manually for each individual image as it becomes
necessray - or just use a ?1, ?2 etc for those.

Enabled by default for CSS and JavaScript links, since those are much
more likely to be changed without having the URL changed.

Cache times aren't increased yet - we'll do that later one we're sure
that all existing caches are expired first.

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