Use landmark regions on columns (#1732)

Closes mastodon#1720

app/assets/javascripts/components/features/ui/components/column.jsx

@@ -64,7 +64,7 @@ const Column = React.createClass({
     }
 
     return (
-      <div className='column' onWheel={this.handleWheel}>
+      <div role='section' className='column' onWheel={this.handleWheel}>
         {header}
         {children}
       </div>

app/assets/javascripts/components/features/ui/components/column_header.jsx

@@ -25,7 +25,7 @@ const ColumnHeader = React.createClass({
     }
 
     return (
-      <div className={`column-header ${active ? 'active' : ''}`} onClick={this.handleClick}>
+      <div aria-label={type} className={`column-header ${active ? 'active' : ''}`} onClick={this.handleClick}>
         {icon}
         {type}
       </div>