Represent multiple cards as one special multi-card #188

When the "Number of Instances" selector for a resource card is greater than 1, we would like to make a special multi-card.

This sub-issue does NOT involve making any major UI changes, but is the precursor to the UI changes discussed in the parent issue.

This involves creating a multi-card class that extends a normal card (should contain a list of resource settings representing the resources within it, this list will then be expanded when sending settings to the backend)