Revert "Revert "PLATFORM-354 - somehow, ParsoidCacheUpdateTask is getting instantiated on wikis that have $wgEnableVisualEditorUI set to false. for now, moving task so it's available anywhere to prevent fatals. @inez - any idea how this can happen?""

This reverts commit 3c0cb1d393579a2dd0039d804e1c7d2da25b73bc.
adding stub class so jobs in the queue do not fail