Refactor Files Header to use `gl-` utility classes

The files header component is currently using custom styles. Let's move away from that and apply gl- util classes on the components.

Make sure to address these too:

• @zcuddy started a discussion: (+1 comment)

  question: Are we able to utilize utility classes to replace this custom style? I'm worried that it doesn't fit our GitLab UI spacing scale

• @zcuddy started a discussion:

  question: Are we able to utilize utility classes to replace this custom style? I'm worried that it doesn't fit our GitLab UI spacing scale

Note

There are a few areas of our app using file-headers, so please make sure to make the changes on ALL of them.