Tracing MVC

What does this MR do?

• add Operations > Tracing nav link
• add Settings > Operations nav link
• add project/tracing page with link to Configure tracing
• add project/edit/operations page to configure tracing URL

CE Backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22767

Tracing landing page:

URL saved, link updated in sidebar:

URL cleared, link resets to Tracing landing page

What are the relevant issue numbers?

Closes #4753 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• 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?