Add hooks for the dashboard and user page sidebar items.

These new hooks allow for plugging content into the dashboard and user
page sidebars, using the new sidebar item registration APIs. They
replace the old DashboardHook and UserPageSidebarHook.

There's technically also a DataGridSidebarItemsHook, but it's more of a
base class. Since there's only two datagrids with sidebars right now,
I'm leaving it undocumented until we start seeing a need to expose it.