Improve outline buttons

At GitLab we are currently discussing how we can improve our outline buttons to:
• make them look and feel more consistent
• make the resting/normal, hover, and active states more noticeable

In this process, the filled buttons will be tweaked as well to keep everything tight.

We encourage you to look at the issue page for more images and context on this work: Improve outline buttons (gitlab-ce#25220)

Give us your feedback so we can improve it together. Thanks!