Bug #1070001 - Re-organize stylus

- Switched from @import to @require.
- Files should now require the files they need rather than relying on an @import in a different file.
- Files in /include should not generate output.
- Tried to break files in main into sensible smaller pieces.
- Ran output file through css validator to try to catch errors, fixed what came up.
Merge pull request #2762 from stephaniehobson/BUG-1070001-reorg-stylus

Bug 1070001: Re-org Stylus