Add missing span info tag around help text.

[Closes #3546]
Style fix for guest checkout label and radio options.

- page: admin/orders/{order.number}/customer
- issue: colon was left out of styling and 'Yes' and 'No' had no
styling nor label.

[Closes #3545]
Bumped CSS Browser Selector to 0.6.2

Also js linted.

[Closes #3543]
expand order number field size to 32
Spree::PermittedAttributes for more extensibility

With existing `ControllerHelpers::PermittedAttributes`, adding
additional permitted attributes can only be done by overriding the
method, and would likely be done with much method aliasing.

This introduces `Spree::PermittedAttributes`, which has as module
attributes the accessible attributes on 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 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.