Closing Card 21. “Feature spec” `belt_and_suspenders` passes.

Clicking the ‘Select’ button in the form now validates inputs before computing selection, which fixes a whole bunch of things.

RSpec status:	 		11 example, 0 failures
Teaspoon status:	 	154 tests: 0 failures, 0 errors, 9 pending, 0 omitted
Custom RuboCop rules:	None
Inline RuboCop disabling:	None (in non-generated code)

Trello cards authorising work:
* https://trello.com/c/X8zeZvdH/21-confirmation-of-selection-endpoint-details
* https://trello.com/c/h6Kw6FkX/14-main-app-landing-page (partial)