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.
So this exposed an ordering issue bug. If this ran first, it might use the wrong environment instance that wasn't cached correctly.