Work item planning views/list page

What does this MR do and why?

Replaces the 'Issues' and 'Epics' links with 'Work Items' in the sidebar

Clicking 'Work Items' takes you to /work_items for the group or project and displays a work items list that contains all work item types.

Toggling the work_item_planning_view feature flag should enable/disable these changes. When the flag is disabled, the legacy issues and epics pages should be accessible like normal.

The pinned 'Issues' and 'Epics' items in the sidebar should be replaced with 'Work Items'.

References

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the work_item_planning_view feature flag
2. Visit any project-level page, and check 'Work items' is visible in the 'plan' section of the sidebar nav instead of 'Issues'.
3. Visit any group-level page, and check 'Work items' is visible in the 'plan' section of the sidebar nav instead of 'Issues' and 'Epics'.
4. If you have 'Issues' pinned in the nav at either the project or group level, 'Work items' should be pinned.
5. If you have 'Epics' pinned in the nav at the group level, 'Work items' should be pinned.
6. Clicking 'Work items' in the sidebar nav should take you to a mixed view of work items in a list.
7. There should be no 'open', 'closed', 'all' tabs and instead you can use a new 'state' filter.

Relates to #512661 (closed)