Implement linked sorting #17
Description
Array-based sort order, i.e. Backbone's default, is unappropriate for collaboration because it won't let us sacrifice the C in CAP theorem.
Units should instead be ordered based on a referencing field. Possibly with fallback to timestamp from #15.
Ideally a change of sort order should only affect the unit that is being moved.
This issue meas we need to reconsider #9, as "up" and "down" based on index won't be supported. Only move.after is native. Up and down must be abstractions on top of that, and assume that local state is ok.