Lineup: Flex follows the latest games

Pick any starters. Flex and SuperFlex always show the latest games; the fixed slots take the earliest.

Demo page. It runs the real shared lineup code (lineup_flex.js, lineup_result_modal.js, the 18-slot model) on a made-up roster and made-up kickoffs. Nothing is sent to MFL. The "Submit" buttons only show the pop-up.