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 do.