Allow users to create blocks of shifts

[tareko]

Users should be allowed to create a block of shifts that all have to be taken / left together.

I imagine this would happen by clicking on the configuration gear (Issue #120) after setting a few shifts into the marketplace, and then making one the parent of the others or such.

Unlike the request below, I don't think the cost/benefit would be worth it for now to allow a team of individuals to pick up the parts, but perhaps this might be a feature later.

Use case scenario:

I had a weekend in October I was hoping to unload. I placed the entire weekend F/S/S on the marketplace. Someone picked up only the Saturday shift. Now I am stuck with the Friday and Sunday.

In this scenario, I am less likely to get rid of the Friday/Sunday combo than I would have been if I kept the weekend off the marketplace, and emailed trade requests. It is more difficult to trade an awkward Friday/Sunday.

This situation would be resolved if I were able to post an entire weekend. This could be such that no part goes until all of it goes (e.g., multiple users can select one or more shifts each), or so that one person must take the entire block.