Avoid submitting forms with MarkdownButton

Add type="button" to the component so that it won't submit if placed inside a form.

Fixes https://github.com/flarum/core/issues/2875

extensions/markdown/js/src/forum/components/MarkdownButton.js

@@ -11,6 +11,7 @@ export default class MarkdownButton extends Component {
     const button = (
       <button
         className="Button Button--icon Button--link"
+        type="button"
         data-hotkey={this.attrs.hotkey}
         onkeydown={this.keydown.bind(this)}
         onclick={this.attrs.onclick}