Ability for users to set their preferred language in admin pages

[bricesanchez]

Because the dashboard was removed, we can't set_locale on admin pages by clicking on a link, like in older versions of Refinery. At this time, we can just pass the params set_locale in the URL.

According to @parndt, we could add a locale column to the User table then each user could set their own preference.

Original talk here : https://github.com/refinery/demo/issues/1#issuecomment-212367150