mediawiki.toolbar: temporary workaround for T113868

mediawiki.toolbar: temporary workaround for T113868

ResourceLoader does not expect the set of files a module depends on to vary by
language, but since I39d8ed4, mediawiki.toolbar does. To fix this, we need to
replace the module_deps table with something less insane. In the interim, we
can avoid needless recurrent cache invalidation by referencing the superset of
all possible image files as the background-image for a dummy selector.

Bug: T113868
Change-Id: I94ced0617814d83beee35d30ff1158709e884a98