change all .nav-links to use flex
place Resolve Discussions above tabs on small screens, and to the right on large

Closes #28909

Closes #31052

The tabs on the merge_requests#new where changing the URL to URLs meant
for merge_requests#show. The URL don't actually exist on the new page,
so the simplest solution is to not update the URL for this page.

Closes #30419

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/29414

Some inner page tabs where not scrollable like the top nav & this caused
them to break onto new lines mobile.

This pages include:

- dashboard/projects
- project -> merge_requests
- search
- project -> milestones
- project -> pipelines

Closes #18853

Adds rel="noopener noreferrer" to all links with target="_blank"

See merge request !2071

Closes #29660