Expose Rails.application.assets_manifest

The manifest instance is only exposed to the view right now, but it'd be nice to access from anywhere. This would add `Application#assets_manifest`.

@rafaelfranca It really bugs me that the state is exposed and stashed on `ActionView::Base.assets_manifest`. Is there a more sane way to scope application config on view instances? Is there any way to access the current "application" from the view?

If theres a view `assets_environment` and `assets_manifest` already, it seems like that should be available on controllers too.