In T267921: Roll out the Toolbox link for URL Shortener in Wikimedia sites, @patilise asked for the sidebar link (implemented as T107101) to be enabled at all Wikimedia wikis. However, as I explained at T267921#6623871, in the current form of the code, $wgUrlShortenerReadOnly needs to be false in order for the sidebar to display. In Wikimedia wikis, that variable is set to true at all wikis but metawiki, in order for throttling to work properly.
We need to introduce a new config variable, to specify the wiki that the shortener lives on, so wikifarms like Wikimedia can have a single shortening wiki used by all other wikis.