Woocommerce-services: Allow the merchant to manually add a new package to the shipment

[DanReyLop]

As part of #449 , in the Packages step, allow the merchant to add a package to the shipment. That means, independently of what the packing algorithm decided, the merchant must be allowed to add a box (from the Package Manager) and put one or several of the order's items in that box.

Note: The UI must allow somehow to add a new type of package. We could simply put a link to the Package Manager, but then we must make sure that the packages list in the Labels UI are updated without reloading the page.

[kellychoffman]

Note: The UI must allow somehow to add a new type of package. We could simply put a link to the Package Manager, but then we must make sure that the packages list in the Labels UI are updated without reloading the page.

This brings up a good point about the shipping labels UIs for when there are zero saved packages. What about doing this?

[DanReyLop]

Looks good. I think it would also be useful to have that link even if the user has saved packages. Something like having a <select> with the last option being Add a package... that takes you to the Packaging Manager.

[kellychoffman]

a <select> with the last option being Add a package...

Great idea. Since it would be strange to have that option be the only option in the select, we should have 'Add a package...' be the last item when there is at least one option present, like in https://github.com/Automattic/woocommerce-connect-client/issues/585#issuecomment-251386056.