Make i18n fallbacks configurable and fallback to the default locale by default in production [#4428 state:resolved]

Allows to configure locale fallbacks through config.i18n.fallbacks. The default setting
config.i18n.fallbacks = true in production.rb will make I18n.t lookup fallback to the
I18n.default_locale if a translation could not be found for the current or given locale.

config.fallbacks = true

config.fallbacks.map = { :ca => :es }
config.fallbacks.defaults = [:'es-ES', :es]

config.fallbacks = [:'es-ES', :es]

config.fallbacks = { :ca => :es }

config.fallbacks = [:'es-ES', :es, { :ca => :es }]
Signed-off-by: NPratik Naik <pratiknaik@gmail.com>