Display a "Name on card" input on payment step

It creates a name column on spree_credit_cards table to insert the data
instead of using the old first_name and last_name fields, fixes #2486
Move payment partial from core to frontend

Plus make it use the name attribute instead of the deprecated first_name
and last_name
Add CHANGELOGs entries for payment name on card feature
there is a conflicting migration in 2-1-stable branche (8de75a5d3d519c5498843edda983702852caaade)
it's order_id ofcourse!
Make Stock::Package more aware of units structure

`add` and `find_item` now might receive a variant and a line item for
cases where they both don't refer to the same product
Grab shipment line items via inventory units

The previous implementations doesn't make sense anymore on the current
split shipments scenario. Can't see a reason why tracking inventory
levels config should influence whether or not the line item should be
returned
Load line items in shipment `manifest`

We still need to figure out a way to lazy load inventory units variant
and line items removing the default scope from Variant
ManifestItem struct for items in manifest

Prefer Struct over OpenStruct (which is slower and masks errors)

Fixes #4039