Handle shipments

[jcoupey]

The frontend code was written before the shipment objects were added to the VROOM API, and was never upgraded to handle them. It would actually be a nice addition to the demo to be able to include shipments.

I see this as a two-step work:

1. adding the notion of shipments in the code logic and have them added to the problem/request when directly submitting a json file as input while adjusting the solution display accordingly;
2. actually allowing defining shipments through the UI.

The latter is probably useful too but more subject to design choices questions. And then also it would raise the question on how to manually define amounts, which we don't do in the UI for now.

[jcoupey]

First step done in #57.

[jcoupey]

Closing here as point 2 is probably not really relevant.