Create "Work Items" Planning Views page

In order to have a place to home the new Planning Views consolidated Work Items list, we should create a new page and update the sidebar navigation to allow users to discover it.

Currently, the sidebar contains 'Issues' at the Project level and both Issues and Epics at the Group level. We should replace these with Work Items, and link to /work_items

As this work will be ongoing, and we do not want to impact existing functionality unecessarily, we will need to introduce a work_items_planning_view feature flag to toggle the Work Items combined list view on and off.

Requirements

• When the work_items_planning_view feature flag is enabled
  • The Issues link in the sidebar is replaced with Work Items in a Project context
  • The Issues and Epics links in the sidebar are replaced with Work Items in a Group context
  • The Work Items sidebar link navigates to /work_items
  • /work_items renders an list of work items which is not filtered by default.
  • If the user has no custom pinned items, the Work Items item is pinned instead of Issues
  • If the user has Issues pinned, the Work Items item is pinned instead.
  • If the user has Epics pinned, the Work Items item is pinned instead.
• When the work_items_planning_view feature flag is disabled
  • The existing Issues and Epics links for Project and Group contexts exist and work as normal
  • The existing pinned item behaviour is retained

Proposal

• Create work_items_planning_view feature flag
• Update sidebar
  • Remove 'Issues' from Project level sidebar
  • Remove 'Issues' and 'Epics' from Group level sidebar
  • Add 'Work Items' to Project and Group level sidebars
  • Use the id for the Issues nav item for the Work Items and Epics nav items to support pinned items. see comment
  • Should only occur when work_items_planning_view flag is enabled
• Create new /work_items web route
  • Render existing WorkItemListApp when visited (with no type filter applied) (see this issue)

Check the docs for how to update the sidebar: https://docs.gitlab.com/ee/development/navigation_sidebar.html#navigation-sidebar