Replace BabelZilla Beta with Transifex

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In #24, I moved this project’s localization from BabelZilla to BabelZilla Beta, a service based on Transifex that specializes in localizing Mozilla’s extension format. However, BabelZilla Beta was abandonware even when I first filed #24. It was enough of an improvement over BabelZilla that I was ready to switch anyways.

The long-term goals are to build an automatic localization workflow and attract localizers for key languages like Russian (#111), French (#112), and Chinese. BabelZilla’s community isn’t what it once was, while Transifex is looking quite healthy.

Since I first looked into Transifex, they’ve added support for Mozilla DTDs, which means their standard workflows should be quite enough for this project. I haven’t used BabelZilla’s other features, like packing up localized XPIs, for years, and I don’t think the localizers used it much either.