We were defining one builder in the `app/lib/` folder and another one
inside a helper module.
So now we're grouping them together. This way we're following the "one
class per file" convention that we follow most of the time. And, by
extracting the `TranslatableFormBuilder` class to its own file, it'll be
easier to add tests for it.
Note that, for consistency, we're renaming the
`TranslationsFieldsBuilder` class so it ends in `FormBuilder`.
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