Scratch pofile from POFileTranslator

[Jeroen T. Vermeulen]

POFileTranslator caches people's contributions to translations. Right now it does this per POFile. That's no longer accurate since a shared message can contribute to multiple POFiles—even ones that don't exist in the database.

There's no perfect way to maintain POFileTranslator, but I think the main problems would be solved by replacing POFileTranslator.pofile with (product, distribution, sourcepackagename, template name, language, variant). That's still ignoring diverged messages; to support those we'd have to add a potemplate FK as well and think about the trigger code some more.

[Данило Шеган (danilo)]

I'd rather if we just go away with the trigger and try to do more of atomic operations in the code. That's yucky as well, but at least more easily controllable with DB transactions. It can also perform better.