add layouts

[dirkraft]

The present value of anvil is simply to save, recall, and share selections of series. Take it to the next (nearby) level of allowing a few basic layouts.

• come up with a rough layout pattern (codewise)
• Start with just one layout: columnar, and user selects number of columns
  • ability to reorder boards on a wall #25 reorder modal must then become a more general layout reorganization modal