This hook in core exists for compatibility reasons and is causing integration problems as we push ahead with desktop refresh leading to an UBN (T252906)
Even worse it has been introduced and run in Vector for some time so the hook runs in two different context.
It works around the fact that no other hooks allow modification of the languages and toolbox menus.
I propose we expand the SidebarBeforeOutput hook to allow for this. See POC patches.
= Acceptance criteria
[x] Toolbox and languages are included in the SidebarBeforeOutput hook https://gerrit.wikimedia.org/r/596756
[x] ShortUrl is updated to use the SidebarBeforeOutput hook to append its toolbar link https://gerrit.wikimedia.org/r/596759
 All extensions have been migrated to SidebarBeforeOutput - T253783 is resolved
 Support for the hook is dropped from Vector without a deprecation notice (given this hook should never have been run inside Vector in the first place). The hook is not deprecated, just its usage inside Vector.