Add settings panel to define dimensions for default parcel to shipping dashboard

[spencern]

With #2992 we now automatically calculate the parcel size for cart items from whichever variant was added to the cart.

We need to add a settings panel within the shipping dashboard that permits defining default parcel sizes for products and variants that don't have dimensions defined.
We should add sensible defaults for the default parcel size so that it's never null which could cause Shippo or other shipment providers to fail to calculate shipping correctly.

This section should permit setting the default parcel size with the fields: width, height, length, and weight.
Other app-wide shipping settings can also live here.

[spencern]

This should default to a flat rate priority box dimensions and a weight of 1lb for all reaction.
This should also permit users to update the default.

[brent-hoover]

@rymorgan Do we actually need design for a settings panel?

[rymorgan]

@zenweasel I think we might at this stage. But something on my upcoming agenda is to work on some standards for settings cards that will enable us to build this w/out design.

But I'm also willing to work backward on something like this if you guys want to take a stab and don't' mind rearranging the UI design once it coded. But let's not do that if it ends up causing us more dev time than it's worth.

[efalayi]

@rymorgan is there a layout for this section that I can use?

[brent-hoover]

@efalayi So these values should be used like this:

1. When shipping is calculated if no values are set (i.e. they are 0) then it should use those values to send to Shippo.
2. If the product is not set and the default is not set it should fall back to the dimensions of a flat rate priority and a weight of 1 so that we never get a situation where we don't have a weight for a product.

[brent-hoover]

from @spencern
USPS Priority Mail Medium Flat Rate Box - 11 1/4" x 8 3/4" x 6"
Not sure what a good default weight would be, maybe 7 or 8 lbs?

Reference for dimensions:
https://www.usps.com/ship/priority-mail.htm

[efalayi]

@zenweasel should the user also have the option of not resetting existing products?
Also, should the minimum values for size be 0 or 1?

[efalayi]

@zenweasel could you please explain this part:

If the product is not set and the default is not set it should fall back 
to the dimensions of a flat rate priority and a weight of 1 so 
that we never get a situation where we don't have a weight for a product.

[brent-hoover]

So this ticket should be just about defining default dimensions for Shippo (or alternately for any shipping plugin) but should not be used for other places such as defaults for the PDP.