separate UI locales from data locales

[Muskworker]

Out of http://guides.rubyonrails.org/upgrading_ruby_on_rails.html :

config.i18n.enforce_available_locales = false

Note that this option was added as a security measure, to ensure user input cannot be used as locale information unless it is previously known. Therefore, it's recommended not to disable this option unless you have a strong reason for doing so.

Ideally we want to make it so we can remove this option, which is currently set because right now the same locale system that allows users to enter dictionary data for arbitrary languages is the same locale system that Rails uses for i18n of the app itself. The former should be allowed to be free/sanitized input, while the latter should be limited to people who can edit the UI translations anyway.

This may involve some overhauls.

[Muskworker]

related #203