Make the music indicator a proper button

src/music/components/indicator.tsx

@@ -23,8 +23,8 @@ class Indicator extends React.Component<Props, {}> {
 
   render() {
     let classes = classNames({
+      btn: true,
       "col-auto": true,
-      "text-center": true,
       fas: true,
       "fa-muted": this.props.muted,
       "fa-play": this.props.playing,
@@ -32,11 +32,12 @@ class Indicator extends React.Component<Props, {}> {
     });
 
     return (
-      <div
+      <button
+        type="button"
         id="playerIndicator"
         onClick={this.click.bind(this)}
         className={classes}
-      ></div>
+      ></button>
     );
   }
 }