Refs #1406; Style(eunis.css) Simplified styles of threat status css declarations

- Set black as the default color for links and text for the threat
  status boxes
- Added hover overrides for classes where the original eea hover color
  is providing a bad contrast ( mostly it affects the threats  with the darker
  backgrounds )
- Set text-decoration to links instead of border bottom since now the links
  contain the threat status text