Closing Card 20; ‘text offset’ fields wired correctly.

Ticks another box in the list on Card 14.

RSpec status:	 		17 example, 0 failures, 1 pending
Teaspoon status:	 	152 tests: 0 failures, 0 errors, 7 pending, 0 omitted
Custom RuboCop rules:	None
Inline RuboCop disabling:	None (in non-generated code)

Trello cards authorising work:
* https://trello.com/c/hpxOqA22/20-error-handling-for-text-offset-edit-box
* https://trello.com/c/h6Kw6FkX/14-main-app-landing-page (partial)