In Ruby 2.4 the `to_time` method for both `DateTime` and `Time` will
preserve the timezone of the receiver when converting to an instance
of `Time`. Since Rails 5.0 will support Ruby 2.2, 2.3 and later we
need to introduce a compatibility layer so that apps that upgrade do
not break. New apps will have a config initializer file that defaults
to match the new Ruby 2.4 behavior going forward.

For information about the changes to Ruby see:
https://bugs.ruby-lang.org/issues/12189
https://bugs.ruby-lang.org/issues/12271

Fixes #24617.

(cherry picked from commit c9c5788a)