Add option to unify display of VE enable preference

Setting $wgVisualEditorUnifiedPreference = true makes the VE enable
preference appear consistently on the preferences page, while still
being backed internally by the old 'visualeditor-enable' and
'visualeditor-betatempdisable' preferences.

(It effectively overrides $wgVisualEditorEnableBetaFeature, which
needs to remain unchanged to use the values from the right preference.)

Bug: T335056
Change-Id: Iad710a769c7176c4673c475d540dff2b7782e159