(CE-829) Always disable RTE in Template and MediaWiki namespaces

There isn't any wikias with wgWysiwygDisabledNamespaces set with these
namespaces enabled, and there's little use case for visual editing in
them, especially as we're moving to VE.
Merge branch 'CE-829' into dev
Merge remote-tracking branch 'origin/dev' into jobqueue-promoteimagereviewtask
change to use the same instantiation method