Add font color contrast to external label in admin area

## What does this MR do?
Adds a darker font color to `.label-default` so that the labels have more contrast

## Are there points in the code the reviewer needs to double check?
~~I used the same font color as the `.badge` on that same page but it was not listed as a variable in `variables.scss`. I'm not sure if both references need to be added as a sass variable or whether they should be replaced with an existing color in the variables list.~~

In reference to [Dmitriy's note about the colors](https://gitlab.com/gitlab-org/gitlab-ce/issues/20114#note_13251919), I changed the color to `$btn-transparent-color` and also took the liberty to change it on the navigation badges as well.

## Why was this MR needed?
Improves existing UI

## What are the relevant issue numbers?
Closes #20744 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-09_at_2.49.04_PM](/uploads/df8f402cc1dd423a2c37c9f4067b95f8/Screen_Shot_2016-08-09_at_2.49.04_PM.png)

After:
![Screen_Shot_2016-08-09_at_3.32.54_PM](/uploads/396f6679618630cab7a40e432f259084/Screen_Shot_2016-08-09_at_3.32.54_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5743