Make modal spinner centered

[MAILPOET-2602]

assets/css/src/components/formEditor/components/_preview.scss

@@ -0,0 +1,7 @@
+.mailpoet_spinner_wrapper {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+
+}

assets/css/src/formEditor.scss

@@ -9,5 +9,6 @@
 @import '../../../node_modules/codemirror/theme/neo';
 @import './components/formEditor/components/form_title';
 @import './components/formEditor/components/sidebar';
+@import './components/formEditor/components/preview';
 @import './components/formEditor/components/block_editor';
 @import './components/formEditor/components/custom_field';

assets/js/src/form_editor/components/preview.jsx

@@ -58,7 +58,9 @@ const FormPreview = () => {
     >
       <Preview>
         {form === null && (
-          <Spinner />
+          <div className="mailpoet_spinner_wrapper">
+            <Spinner />
+          </div>
         )}
         {form !== null && (
           <div>