page.tmpl: Add Cache-Control must-revalidate to ensure that users (especially of...

page.tmpl: Add Cache-Control must-revalidate to ensure that users (especially of Firefox) see fresh page content.

Since Firefox version 3, it's done aggressive caching of visited pages, and
does not, by default, check if the cached content is still valid when
reloading or revisiting a page. By default, Firefox seems to not re-contact
the web server at all. Compare with eg, Epiphany and Chromium, which appear
to always check, and get back a 304 when the page is unchanged.

This header makes Firefox do the right thing, at least for html files. It
still over-caches if css, javascript, images, etc, are changed.