(bug 33513) Define altUploadForm in $wgUploadWizardConfig

Unfortunately, with the current format of URLs accepted by the
UploadWizard's altUploadForm, there is no (easy) way to pass on
user's language; providing links with ?uselang=xx leads
to non-existing pages on Commons, which isn't particularly
helpful.

For now, let's just serve working links.

Bug: 33513
Change-Id: Ifda8b61a8ae5ff04de5214925c23869e47e3bdd3