Moved i18n to shared
Moved i18n to shared

refs https://github.com/TryGhost/Ghost/commit/90ca836cb64a97b3b143b876a11b8d084bfd8d3c

- i18n is used everywhere but only requires shared or external packages, therefore it's a good candidate for living in shared
- this reduces invalid requires across frontend and server, and lets us use it everywhere until we come up with a better option

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