Bug 48040: Check for filename conflicts before upload

Checks eg 'Foo.jpg', 'Foo 2.jpg', 'Foo 3.jpg' etc in sequence,
and uploads using the first title that is found to be unused
by an image on the target wiki.

Still possible to get conflicts from *during* the upload, but
they'll be much less likely.

https://bugzilla.wikimedia.org/show_bug.cgi?id=48040
https://trello.com/card/ios-handle-filename-conflicts-transparently/51830a8104d981c70c0012ce/29