Improve extensions page

- Allow extensions to define an icon in their flarum.json
- Show a "please wait" modal when enabling/disabling an extension
- Styling tweaks

js/lib/components/Modal.js

@@ -28,13 +28,15 @@ export default class Modal extends Component {
     return (
       <div className={'Modal modal-dialog ' + this.className()}>
         <div className="Modal-content">
-          <div className="Modal-close App-backControl">
-            {Button.component({
-              icon: 'times',
-              onclick: this.hide.bind(this),
-              className: 'Button Button--icon Button--link'
-            })}
-          </div>
+          {this.isDismissible() ? (
+            <div className="Modal-close App-backControl">
+              {Button.component({
+                icon: 'times',
+                onclick: this.hide.bind(this),
+                className: 'Button Button--icon Button--link'
+              })}
+            </div>
+          ) : ''}
 
           <form onsubmit={this.onsubmit.bind(this)}>
             <div className="Modal-header">
@@ -50,6 +52,15 @@ export default class Modal extends Component {
     );
   }
 
+  /**
+   * Determine whether or not the modal should be dismissible via an 'x' button.
+   *
+   * @return {Boolean}
+   */
+  isDismissible() {
+    return true;
+  }
+
   /**
    * Get the class name to apply to the modal.
    *