Fix active nav item for Service Desk page

What does this MR do and why?

Fixes #412648 (closed) by making the active_route for the Issues page more specific. Before, its controller: 'projects/issues' pattern also matched the Service Desk controller action, so it highlighted wrongly itself when it shouldn't.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Go to any project.
• In the left sidebar, go to Monitor/Service Desk.
• Check the active item of the sidebar. Only Service Desk should be active. When you collapse the Monitor section, it should show as active.
• The Issues item in Plan should not show as active.

Also test that Issues does show as active where it should:

• Navigate to Plan/Issues. Should show as active.
• Navigate to an issue's show view. Should show as active.
• Navigate to the new issue form. Should show as active.
• Can you think of any other controller action that should show as active??

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.