Rename div as main
Rename div as main

Change <div> with class="layout__columns" to <main>. Revert previous
<main> back to <div>. Issue #1200

jwang1919/popcode,popcodeorg/popcode,jwang1919/popcode,popcodeorg/popcode,outoftime/learnpad,popcodeorg/popcode,popcodeorg/popcode,outoftime/learnpad,jwang1919/popcode,jwang1919/popcode