Add a $wgUploadMissingFileUrl entry for enwiki

So as to point wiki links to missing files
(a.k.a. red links) to Special:Upload instead of
Wikipedia:File_Upload_Wizard which is inherited
through $wgUploadNavigationUrl.

This patch basically reverts this part of English
Wikipedia configuration to how it was before
I38a0679 (or bug 42263).

Change-Id: I59575e79a7663ab24773f58d51d4e4c7479425b1
Merge "Add a $wgUploadMissingFileUrl entry for enwiki"