Bug 869516 - base styles for older versions of IE

**DO NOT MERGE** - We need to do some testing of major pages, especially those that need to perform well in IE like Firefox product pages, /new, /fx, etc.

After burning a lot of time updating a bunch of individual pages, I changed my approach and arrived at something much simpler by adding a new block in the base templates. The existing <code>site_css</code> block and the new <code>page_css</code> block are both hidden from old IEs but the <code>extrahead</code> block is not. This means most pages that already use <code>extrahead</code> to include page-specific styles continue to work as they have (since they've already been tested with old IEs).

Future pages that need page-specific styling can now use the <code>page_css</code> block to hide those styles from oldIE and you usually won't need to bother testing. Old IE gets the basic styling and doesn't see your new CSS at all. Use good markup and your content should degrade to something perfectly readable and presentable, just not fancy.

Note that the JavaScript blocks are NOT hidden from old IEs, in order to avoid breaking a lot of pages already built and tested for them. In the future, you can include conditional comments *inside* the <code>site_js</code> or <code>js</code> blocks to let old IE degrade without any fancy JS, but I didn't want to make that change global in the base templates (I tried it; lots of stuff broke).

Script-heavy pages like /firefox/partners and /firefox/os will need some attention before this can merge since the JS on those pages makes some assumptions about the CSS. We can either ensure IE gets the styling (by putting it in <code>extrahead</code>) or we might be able to just hide the JS from oldIE on those pages and let them degrade.

Some pages might have content hidden in the page-specific CSS that is then made visible by JS when needed (stuff like modals and close buttons). If you're explicitly hiding elements in your page styles just remember that content will be visible in old IEs since they don't get your page-specific CSS. oldIE.less does include the same <code>.hidden</code> class from sandstone so you can still use that to visually hide content and it'll be hidden in IE as well.