Remove owner permissions check from POTemplate.newPOFile

POFiles are created through POTemplate.newPOFile. This method checks whether the person creating the POFile would have editing permissions on it. If so, the requester becomes the owner. If not, rosetta_admins becomes the owner.

This triggers a permissions check that has tripped up us several times on database permissions for rarer cases. It'd be more transparent if this check were done at the call site.

Moreover, POFile.owner doesn't seem to be used for anything meaningful, so we may not need it at all. The only thing it seems to do is give the owner editing permissions—which in the current code they must have had anyway to have become the owner in the first place. We could drop it, or allow it to be NULL and make that the default.