Automatically rename "messages.po" uploads

[Jeroen T. Vermeulen]

We could save some time and mistakes on manual approval if uploaded files called "messages.po" were automatically renamed to "messages.pot" either during upload or by the approval script. Upload is probably better; we can show a warning for it there.

I believe most mistakes with filename extensions in uploads are templates being uploaded with this exact name. But once we can deal with this, we may run into other typical template.po names that we'd want to deal with in the same way.