Add transifex config and locale files

[dominik-zeglen]

Title is self-explanatory.
Resources can be now translated here: https://www.transifex.com/mirumee/saleor-1/translate/#pl/storefront

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. The changes are tested.
4. The code is documented (docstrings, project documentation).
5. Changes are mentioned in the changelog.