Spree::PermittedAttributes for more extensibility

With existing `ControllerHelpers::PermittedAttributes`, adding
additional permitted attributes can only be done by overriding the
method. To be done safely in extensions, this would rely on a lot of
method aliasing.

This introduces `Spree::PermittedAttributes`, which has as module
attributes for the the accessible attributes on the spree models. This
is easily modifiable by spree applications or extensions.

    Spree::PermittedAttributes.line_item_attributes << :my_custom_attribute

`ControllerHelpers::StrongParameters` contains the same methods as
before but mostly delegates to `Spree::PermittedAttributes`. The
controller helpers also describe the allowed nested attributes.

For more complex customization (ex. permissions depending on role),
controllers can still override `permitted_attributes` with
`SimpleDelegator` (or similar) or override `permitted_*_attributes`
methods themselves to modify the attributes on a per request basis.

Thanks to Hates, Radar, huoxito, and jstrong in #spree for quick
discussion.