Redirect from issues and epics route to work items route

Once the "Work Items" nav item is added behind a feature flag in #512661 (closed), we will need to ensure the /issues and /epics routes that it replaces redirect correctly to /work_items.

old route	new route	mr
/issues	/work_items?type=issue	!186431 (merged)
/epics	/work_items?type=epic	!186976 (closed)

As the existing Issues and Epics list pages populate the URL with several filter and sort related search parameters, we should ensure those are retained when redirected to the newer work items list.

Requirements

• Must redirect visits to /epics to /work_items and add type=epic to the URL search parameters
• Must redirect visits to /issues to /work_items and add type=issue to the URL search paramters
• Must not remove existing URL search parameters from the original URL (e.g. /epics?sort=created_date should become /work_items?type=epic&sort=created_date)
• Must not redirect if the work_item_planning_view feature flag is disabled.

Note

There may be a small amount of frontend required if breadcrumbs on the /issues/:iid, /epics/:iid, and /work_items/:iid pages start behaving strangely. It'd be best to split that into a separate followup issue if required.