Redesign unit picker

[djwhitt]

The table based design is getting a bit unwieldy with the full set of units. I think something that displays icons for each unit and lets you select to view details is probably what we want instead. We might further subdivide or provide filtering by unit categories (armored, ranged, etc.). We should also provide a mechanism for comparing, perhaps something where you select multiple units and it displays both their stats side by side.