Duplicate variants when duplicating products.

Cherry-pick of 69f785d732e2f7a3ad129095044413a58a97e32f

Fixes #4339
enable variant view in api. variants list is already accessible, and show method already defined on variants
controller, so no reason not to expose this

Fixes #4336
Ensure prices with currencies displayed do not break variant radio buttons on cart form

Fixes #4342

Conflicts:
	frontend/app/views/spree/products/_cart_form.html.erb