Automate /locales/ page via a configurable list in settings

[alexgibson]

We added a new /locales/ page in #6397, which is currently a hard-coded HTML template, containing the currently active locales for which we have home page transaltions. It would be great to make updating this list slightly more configurable.

While I don't think it necessarily needs to be 100% automated, having a configurable list in settings might be a happy medium. Paul suggested something like the following may work, which we could add to product-details and query in the template:

LOCALES_PAGE = {
    'america': ['en-US', 'en-CA', 'ex-MX', ...],
    'apac': [...],
}

/cc @pmac

[alexgibson]

/cc @rraue