Magic Partitioning

Right now we have the nuclear option of blowing a way a whole disk, or the non-magical CustomPartitioning view where you can select every partition and tell it what to do.

We want to add a smarter partition view here that handles the various possibilities listed in the flow chart at https://github.com/elementary/installer/wiki#scary-but-hopefully-accurate-chart. This includes differentiating between a clean install, upgrade, refresh, dual boot, etc.