Closing Card 19; ‘node index’ fields wired correctly.

We’ve extracted several classes from (working) `NodeIndexValidator` code that don’t have spec coverage yet; that ought to get fixed Real Soon Now™.

Ticks another box in the list on Card 14.

RSpec status:	 		17 example, 0 failures, 1 pending
Teaspoon status:	 	147 tests: 0 failures, 0 errors, 4 pending, 0 omitted
Custom RuboCop rules:	None
Inline RuboCop disabling:	None (in non-generated code)

Trello cards authorising work:
* https://trello.com/c/VB5QzwgS/19-error-handling-for-node-index-edit-box
* https://trello.com/c/h6Kw6FkX/14-main-app-landing-page (partial)