Added membersEnabled config to lexical editor

refs https://github.com/TryGhost/Team/issues/3456

- adds a boolean config whether members signup is enabled so that we can
  hide the signup card option from the editor if it's not enabled.

ghost/admin/app/components/koenig-lexical-editor.js

@@ -274,6 +274,7 @@ export default class KoenigLexicalEditor extends Component {
             feature: {
                 signupCard: this.feature.get('signupCard')
             },
+            membersEnabled: this.settings.get('membersSignupAccess') === 'all',
             siteTitle: this.settings.title,
             siteDescription: this.settings.description
         };