Set $wgUploadNavigationUrl for plwikisource

So as to direct future uploads to Commons while
still letting more experienced users upload files
locally.

See bug for URL with on-wiki consensus for this
change.

Bug: 68191
Change-Id: I517f96bf4e0c2559cccdb6f489386739e0a81911
Merge "Set $wgUploadNavigationUrl for plwikisource"