A common usage of I18n with different locales is to create some around
callback in the application controller that sets the locale for the
entire action, via params/url/user/etc., which ensure the locale is
respected for the duration of that action, and resets at the end.

Devise was not respecting the locale when the authenticate failed and
triggered the failure app, because that happens in a warden middleware
right up in the change, by that time the controller around callback had
already reset the locale back to its default, and the failure app would
just translate flash messages using the default locale.

Now we are passing the current locale down to the failure app via warden
options, and wrapping it with an around callback, which makes the
failure app respect the set I18n locale by the controller at the time
the authentication failure is triggered, working as expected. (much more
like a normal controller would.)

I chose to introduce a callback in the failure app so we could wrap the
whole `respond` action processing rather than adding individual `locale`
options to the `I18n.t` calls, because that should ensure other possible
`I18n.t` calls from overridden failure apps would respect the set locale
as well, and makes it more like one would implement in a controller. I
don't recommend people using callbacks in their own failure apps though,
as this is not going to be documented as a "feature" of failures apps,
it's considered "internal" and could be refactored at any point.

It is possible to override the locale with the new `i18n_locale` method,
which simply defaults to the passed locale from the controller.

Closes #5247
Closes #5246

Related to: #3052, #4823, and possible others already closed.
Related to warden: (may be closed there afterwards)
https://github.com/wardencommunity/warden/issues/180
https://github.com/wardencommunity/warden/issues/170