Initialise component state in init() instead of constructor

This allows component state to be overridden via monkey-patch. ref #246

js/lib/components/AlertManager.js

@@ -6,9 +6,7 @@ import Alert from 'flarum/components/Alert';
  * be shown and dismissed.
  */
 export default class AlertManager extends Component {
-  constructor(...args) {
-    super(...args);
-
+  init() {
     /**
      * An array of Alert components which are currently showing.
      *

js/lib/components/Checkbox.js

@@ -14,9 +14,7 @@ import icon from 'flarum/helpers/icon';
  * - `children` A text label to display next to the checkbox.
  */
 export default class Checkbox extends Component {
-  constructor(...args) {
-    super(...args);
-
+  init() {
     /**
      * Whether or not the checkbox's value is in the process of being saved.
      *

js/lib/components/Modal.js

@@ -9,9 +9,7 @@ import Button from 'flarum/components/Button';
  * @abstract
  */
 export default class Modal extends Component {
-  constructor(...args) {
-    super(...args);
-
+  init() {
     /**
      * An alert component to show below the header.
      *

js/lib/components/ModalManager.js

@@ -7,9 +7,7 @@ import Modal from 'flarum/components/Modal';
  * overwrite the previous one.
  */
 export default class ModalManager extends Component {
-  constructor(...args) {
-    super(...args);
-
+  init() {
     this.showing = false;
     this.component = null;
   }