Default timezone is better to be Rails.application.config.time_zone

Created by: nao58

Hi,

I've just realized this change https://github.com/thoughtbot/administrate/pull/1009 and in trouble now. My datetime field's timezone is in local on input form, utc on view after the gem has updated. To solve this, I should add with_options to ALL current datetime fields.

I think the default timezone is better not to be fixed UTC but rails default configuration, something like this: https://github.com/thoughtbot/administrate/blob/29166a33cdd4f09df3fe8d7aa330503e2d90343d/lib/administrate/field/date_time.rb#L27

def timezone
  options.fetch(:timezone, Rails.application.config.time_zone || "UTC")
end

How do you find it?