Readjust markup of project overview nav

What does this MR do?

Adds another container inside nav.project-stats so that the border reaches the end of the screen while still having the controls fixed/fluid depending on user settings.

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

If the markup changes causes catastrophic failure.

Why was this MR needed?

Speaking with @cperessini over in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20932#note_92385424, the nav border should extend across the screen.

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #51715 (closed)