Fix text colours on discussion list screen:

- narrow down selector for white text for spans to table heading anchors only
- narrow down selector for black text for .sort-desc elements to
  discussion-list only (was causing .sort-desc elements on other list
screens to be black as well)
- narrow down the other selector for .sort-desc to .summary-header only
  - has no effect on screens but might help avoid similar copy+paste errors in future!
https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1222689