Update line_item partial for latest spree changes

Basically just re-indented the whole block within the new
`fields_for` block, aside from applying recent changes to
the original view that were preventing adding an item 
to the cart
Set quantity default value to 1

This change was made here: https://github.com/spree/spree/commit/2717e7574d2c230ec113b5f4b19db0bc4f96f1cf

(seems like) they're assuming a value, so we better go with the flow here.