Make the inserter the same width as the other blocks

[MAILPOET-2813]

assets/css/src/components-form-editor/_block-editor.scss

@@ -45,3 +45,8 @@
 h2 {
   font-size: 1.7em;
 }
+
+.block-list-appender .block-editor-inserter {
+  margin: 0 auto;
+  max-width: 580px; // the same as .wp-block
+}

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

@@ -51,6 +51,7 @@ export default () => {
   const editorSettings = {
     showInserterHelpPanel: false, // Disable TIPs section in add block pop up
     mediaUpload: canUserUpload ? uploadMedia : null,
+    maxWidth: 580,
     __experimentalFetchLinkSuggestions: fetchLinkSuggestions,
   };