Epic tree counts don't update when adding an existing epic

When adding an existing epic to an epic, the number of issues within the newly added epic appear as 0:

However, the epics do have issues inside of them:

This is fixed once refreshing

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Solution proposal

We have separate actions for adding and creating items. When creating items we already call fetchItems, we should just do the same for the addItem (and removeItem) action.

See #335311 (comment 1216179133) for the expected result.

Breakdown

1. Refetch child items on adding and removing items
2. Refetch counts on move (this requires a bit more work with the loading indicator re-rendering)
   • Advantage: If we get rid of the loading indicator on re-fetching, we can do the same for "add new" and gives us a much better experience