VE-1605: Introduced new global variable: $wgVisualEditorNeverPrimary

This new global variable disallow VisualEditor from ever appearing as a main/default/primary editor.
At the same time I'm removing old wgForceVisualEditor variable as not needed anymore.

https://wikia-inc.atlassian.net/browse/VE-1605
It's displayed on Special:Preferences in "editing" section (in specific case - explainer further in the ticket). If you want to test it make sure to rebuildmessages (it does not display otherwise)