Docgen: Add a way to override parameters to makeStaticLoader.

When running from Jenkins, we publish only the docs/ directory.
So the live previews are failing as there is no ../modules to be
found.

This will allow Jenkins to instead pass a different --ve-path.

Also, needed to fix the eval in PHP to not use vePath directly.
The vePath is the path for the web browser to the module directory
not for the server-side.

On the server-side we can just use __DIR__ as maintenance/
is always next to /modules and /VisualEditor.i18n.php.

Change-Id: I02101454a1977938bf861425db2141bdb446725b