Seperate extension strings from website strings to be translated

Our translations are managed by weblate now, and we no longer have access to a _completed branch that contains almost fully translated locales. This requires us to make a few changes to how we decide which locales are complete enough to include.

Because the website and extension strings are all in the same component, the recent website update has caused us to lose fully translated locales for the web extension. If we separate the extension and website strings, changes to one won't impact the availability of locales in another. This will require some code changes on our end.