Refactor extraModules to $wgVisualEditorPreferenceModules

Instead of having a hard-coded array of preference-triggered modules
that we need to maintain in multiple places, manage this data in one
place in a configuration variable, and explicitly declare the preference
and module names rather than interpolating both of them from a name.

This allows extensions to add preference-triggered modules, and it
allows them to have preference names and module names that aren't
prefixed with 'visualeditor-enable-' and 'ext.visualEditor.' respectively.

Change-Id: I9eb14349cd39125d5c11ffb44c962cc5febb6ba0
Merge "Refactor extraModules to $wgVisualEditorPreferenceModules"