Update sidebar navigation counts consistently in response to page actions

Both the old navigation and new navigation update some counts, but not all. What updates and what doesn't isn't consistent across different types of countable things, like Issues, Merge Requests, To-Dos, and Epics.

Description	Old nav	New nav
Neither update the To-Dos count in the nav item itself	simplescreenrecorder-2023-10-27_16.12.10	simplescreenrecorder-2023-10-27_16.10.57
Old navigation updates the issue count in the nav item when an issue is opened or closed, but not the new navigation	simplescreenrecorder-2023-10-27_16.14.09	todo_updates

The reason for this inconsistency in both cases is presumably because these updates have been implemented ad hoc, rather than in one consistent way.