Resolve "Tracking navbar links not behind dropdowns in Snowplow"

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7787 we implemented Snowplow tracking for clicks on navbar dropdowns that do not browse away from the page.

This MR adds the query parameter nav_source=navbar to links in the navbar (as well as links behind a navbar dropdown) that navigate away from the current page.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8808

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #8808 (closed)