Revert r39021 "Tweak css load order (broken for extensions since r38116)"

Revert r39021 "Tweak css load order (broken for extensions since r38116)"
This regresses us back from using <link rel="stylesheet">, which we want, to using @import, which we don't.
There are a few reasons we don't want it:
* IE and Firefox at least don't save @import'ed stylesheets when you save a web page as bundled HTML + resources, which is annoying
* Allegedly, browsers may be more efficient about the order of things they load when using <link>, however I haven't tested this
* the CSS media markers may be different, we're trying to make that consistent...
* Trying to get things to use a consistent interface so we can switch to merging and compressing all these files in a sane fashion