Include I18n::Fallbacks and set the default I18n locale to our default locale.

Thus, when a translation is missing in the current locale, fall back to the
default locale. See https://github.com/svenfuchs/i18n/wiki/Fallbacks for more
info and configuration settings available to users.

Also, this change adds a "t" helper that proxies to I18n.t, just like Rails,
for more concise translating.