Fix accessibility of column headers

As a screen reader user new to Mastodon, I encountered the following issues with the column headers as designed:
 * Jumping between them was difficult. FOr instance, passing my home timeline to reach notification settings was difficult to impossible, especially considering infinite scrolling.
 * There doesn't appear to be any means for triggering the control via the keyboard. the `titleClick` handler only responds to mouse clicks.
 * I didn't even realize there was a Settings toggle until I made this change.

Thanks for using ARIA in your designs. It's a huge help. But adding a `button` role doesn't add keyboard handling and other button behavior. Also, because the role was on the heading container, it obscured the controls within the container itself. This fix resolve that. It also exposes the headings as headings rather than buttons, enabling skipping columns by using screen readers' heading navigation commands.

Since I myself am blind, if this fix requires additional visual styling, I'd like help applying that so it can be merged. I'd consider it an essential accessibility fix for my and other blind users' existence on the platform. Thanks!

app/javascript/mastodon/components/column_header.js

@@ -23,7 +23,6 @@ export default class ColumnHeader extends React.PureComponent {
     icon: PropTypes.string.isRequired,
     active: PropTypes.bool,
     multiColumn: PropTypes.bool,
-    focusable: PropTypes.bool,
     showBackButton: PropTypes.bool,
     children: PropTypes.node,
     pinned: PropTypes.bool,
@@ -32,10 +31,6 @@ export default class ColumnHeader extends React.PureComponent {
     onClick: PropTypes.func,
   };
 
-  static defaultProps = {
-    focusable: true,
-  }
-
   state = {
     collapsed: true,
     animating: false,
@@ -68,7 +63,7 @@ export default class ColumnHeader extends React.PureComponent {
   }
 
   render () {
-    const { title, icon, active, children, pinned, onPin, multiColumn, focusable, showBackButton, intl: { formatMessage } } = this.props;
+    const { title, icon, active, children, pinned, onPin, multiColumn, showBackButton, intl: { formatMessage } } = this.props;
     const { collapsed, animating } = this.state;
 
     const wrapperClassName = classNames('column-header__wrapper', {
@@ -135,11 +130,13 @@ export default class ColumnHeader extends React.PureComponent {
 
     return (
       <div className={wrapperClassName}>
-        <h1 tabIndex={focusable ? 0 : null} role='button' className={buttonClassName} aria-label={title} onClick={this.handleTitleClick}>
-          <i className={`fa fa-fw fa-${icon} column-header__icon`} />
-          <span className='column-header__title'>
-            {title}
-          </span>
+        <h1>
+          <button  className={buttonClassName} onClick={this.handleTitleClick}>
+            <i className={`fa fa-fw fa-${icon} column-header__icon`} />
+            <span className='column-header__title'>
+              {title}
+            </span>
+          </button>
 
           <div className='column-header__buttons'>
             {backButton}