Fix compose form submission reloading web interface (#19762)

* Fix compose form submission reloading web interface

Fix regression introduced by #19742

* Fix various compose form buttons being handled like submit buttons

* Fix coding style issue

* Fix missing onClick prop check

app/javascript/mastodon/components/button.js

@@ -16,8 +16,12 @@ export default class Button extends React.PureComponent {
     children: PropTypes.node,
   };
 
+  static defaultProps = {
+    type: 'button',
+  };
+
   handleClick = (e) => {
-    if (!this.props.disabled) {
+    if (!this.props.disabled && this.props.onClick) {
       this.props.onClick(e);
     }
   }