Bug 1070001: Re-org Stylus

- Switched from @import to @require.
- Files should now @require the files they need rather than relying on an @require in a parent include.
- Files in /include should not generate output.
- Tried to break files in main into sensible smaller pieces.
- Renamed /main to /base to better communicate its function (hopefully this will do away with my brain tick that thinks the global nav stuff belongs in this folder).
Turns out we need mixins but not vars