added lighter count badge background-color for on white backgrounds

What does this MR do?

It lightens the default count badge background color for count badges on white backgrounds and adds a class to have the current bg-color for count badges for in the project nav.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Currently we make no difference if a count badge is on a white background or on a grey background. This MR fixes that, while retaining the old badge color within the grey project navigation bar.

was needed for this MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754#note_19222345

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

none