Allow setting available locales for each store

[tvdeyen]

Solidus 2.2 (latest master while creating this issue) has dropped the general settings page in favour of settings for individual stores.

This gem only allows to set one global setting. We should add support for locales settings for each individual store as this is one of the general purposes of running multiple stores.

[jhawthorn]

As a temporary quick fix, I've done #83, but this would be an even better solution.

[DanielePalombo]

I'm on it

[aitbw]

I think this is safe to close as per #95 and solidusio/solidus#2674