Followup 1cf5a6e: use $wgResourceBasePath to refer to resources/assets, not $wgScriptPath

Bonus: actually make $wgResourceBasePath default to $wgScriptPath, rather than
special-casing it in ResourceLoaderFileModule.

Change-Id: I608435cef00d3e77a5bbdb0a0122d3e7e1a4eb78