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
Merge pull request #5296 from Wikia/VE-1605

VE-1605: Introduced new global variable: $wgVisualEditorNeverPrimary