client/posts: clear messages before submit

Saving the post twice resulted in two 'Post saved!' messages.

client/js/controllers/post_controller.js

@@ -102,6 +102,7 @@ class PostController {
 
     _evtPostEdit(e) {
         this._view.sidebarControl.disableForm();
+        this._view.sidebarControl.clearMessages();
         const post = e.detail.post;
         if (e.detail.tags !== undefined) {
             post.tags = e.detail.tags;

client/js/controls/post_edit_sidebar_control.js

@@ -182,6 +182,10 @@ class PostEditSidebarControl extends events.EventTarget {
         views.disableForm(this._formNode);
     }
 
+    clearMessages() {
+        views.clearMessages(this._hostNode);
+    }
+
     showSuccess(message) {
         views.showSuccess(this._hostNode, message);
     }