site setting for category colors

- [x] category colors is now a site setting
- [x] default category colors are [DESIGNER APPROVED](https://github.com/discourse/discourse/pull/462#issuecomment-14991562)
- [x] differentiated foreground and background colors
- [x] only **black & white** is available for foreground colors
- [x] localized explanatory strings

*original pull request: #476*