Update subscription contract lines.

Discount codes: It's an array type, but we only accept one discount code currently. Important note: If no discount code is provided, any existing discount codes will be removed from the subscription.

Editing lines associated with a build a box: When editing lines associated with a build-a-box make sure to pass in, and update when needed, the custom attributes. If you don't provide the correct custom attributes you are at risk of breaking the box functionality. When unsure, pass along the same exact custom attributes as were previously on the line. When adding a new line to a box, make sure to provide the same custom attributes that exist on each other line in the box. All lines in a Dynamic Build-a-box will contain the same custom attributes as each other.