Fix numbering of sequences and make it less able to bring down form preview

[chrisekelley]

Sequence numbering depends upon 0-based number:

0,1,2

instead of

1,2,3

which is not intuitive for most users. We should change this in the code to accept 1-based numbering.

Change the calc of items to enable 1-based orders, so you can use 1,2,3 instead of 0,1,2

in tangy-form-reducer:

      currentSequence.forEach((e)=>tempItems.push(newState.items[e]))
// modify to 
      currentSequence.forEach((e)=>tempItems.push(newState.items[e-1]))

Also should modify the instruction to illustrate this scheme.