There was an edge case where a user could configure a locale and then
the application would change the locales to that one would no longer be
available. In that case, we were getting a `I18n::InvalidLocale`
exception when accessing the subscriptions page.
So now, we're defaulting to `I18n.locale`. Note we're using
`I18n.locale`instead of `I18n.default_locale` because `set_user_locale`
is called inside the `switch_locale` block in `ApplicationController`,
which already sets `I18n.locale` based on `I18n.default_locale`.
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