Fixed "Send invitation" button needing to be clicked twice

refs https://github.com/TryGhost/Ghost/issues/7131
- adds `disableMouseDown` flag to `{{gh-task-button}}`
- uses flag for the send invite button so that the focus-out validation on the e-mail input is not triggered before the click event is triggered

ghost/admin/app/components/gh-task-button.js

@@ -135,6 +135,14 @@ const GhTaskButton = Component.extend({
         return false;
     },
 
+    // mouseDown can be prevented, this is useful for situations where we want
+    // to avoid on-blur events triggering before the button click
+    mouseDown(event) {
+        if (this.disableMouseDown) {
+            event.preventDefault();
+        }
+    },
+
     // when local validation fails there's no transition from failed->running
     // so we want to restart the retry spinner animation to show something
     // has happened when the button is clicked

ghost/admin/app/templates/components/modal-invite-new-user.hbs

@@ -54,5 +54,6 @@
         successText="Sent"
         task=sendInvitation
         class="gh-btn gh-btn-green gh-btn-icon"
-        disabled=fetchRoles.isRunning}}
+        disabled=fetchRoles.isRunning
+        disableMouseDown=true}}
 </div>