Fix duplicate 'Work items' nav items

In Work item planning views/list page (!180972 - merged), frontend logic was added to remove duplicate nav items based on the nav item text. This was done to work around a limitation in how the nav changes described in Navigation update - Product Planning request: r... (#513092 - closed) and Create "Work Items" Planning Views page (#512661 - closed) were implemented.

We originally implemented the logic for migrating the nav items from 'Issues' and 'Epics' to a single 'Work items' item by renaming the existing nav items. This meant that if the user had one or the other pinned, 'Work items' would appear pinned in its place. However, if the user had both 'Issues' and 'Epics' pinned, two 'Work items' would appear. Frontend logic was added to ensure no duplicate items can appear.

Unfortunately it was not known that there were existing nav items with identical names already, and if those were pinned, one of them would not appear. This would cause unexpected behaviour.

Acceptance Criteria

• Find a way to de-duplicate items that are from the same sub-group only, rather than from all nav items.
• OR, find another way to handle backwards compatibility of pinned items when consolidating two legacy items into one new one.