chore(TripPlan.InputForm): change modes type from enum to map

[thecristen]

While polishing up the mode selector I struggled to reconcile the current form field values (for modes, initially a list of atoms, e.g. [:RAIL, :BUS]) with how the default checkboxes work (each setting a true/false value). Rather than devise some new checkbox group component, I decided to keep the traditional checkboxes and change the form schema. So now modes is a map, where each key is a mode and each value is a boolean. It works great.

I also moved some display logic outside of the form live component. This let me render the selected modes summary outside the form like so :)

Finally added some missing tests around form validation mostly.