OtherProjectsSidebarGenerator in Wikibase client is what creates the sidebar (there's more code needed to display it etc., but that one does most of the heavy lifting).

Problems:

• Generating the sidebar requires loading all Sites from CACHE_ACCEL (we might already do this on every placeholder view, don't know for sure).
• OtherProjectsSidebarGenerator creates the sidebar by getting the Item that is linked with the current page, that obviously wont work for ArticlePlaceholders, so further refactoring will be required there.

Patch: https://gerrit.wikimedia.org/r/302728