Selection start and end selectors now presented as “select” control.

This means, among other things, that Card 18’s task of implementing error handling for Selector edit boxes has been obsoleted; selections presented to the user are guaranteed to be valid.

Closes Cards 18 and 23, and ticks another box in the list on Card 14.

RSpec status:	 		17 example, 0 failures, 1 pending
Teaspoon status:	 	124 tests: 0 failures, 0 errors, 1 pending, 0 omitted
Custom RuboCop rules:	None
Inline RuboCop disabling:	None (in non-generated code)

Trello cards authorising work:
* https://trello.com/c/iweMpez5/23-change-selector-edit-boxes-to-list-controls
* https://trello.com/c/ZbFw6WNN/18-error-handling-for-selector-edit-box
* https://trello.com/c/h6Kw6FkX/14-main-app-landing-page (partial)