Minor improvements to Issues UX

I put a lot of time into making the Label icons and colors nice, but threw in the towel after a few colors, and am still not sure about a few of the labels…

- [ ] Decide whether the _Effort_ icons are what we want (some form of progressing smileys could be a reasonable alternative)
- [ ] Finalize background colors for _Needs_, _Area_, and _State_ categories. Use [this site](http://colorsafe.co/) to determine good candidates for background colors (use prominent icon colors and see what it suggests)
- [ ] Where does _testing_ go as a category? _Continuous integration_? _Tools_, _Automation_, and _Docker_ also seem to have some overlap