Minor Portal settings UX improvements

refs. https://github.com/TryGhost/Team/issues/1813

- All the groups of the Portal settings accordion was closed by default. Opening the first group when showing the modal makes the workflow faster.
- The top heading on the content area had a white background which made the screen visually a bit cluttered.

ghost/admin/app/components/modal-portal-settings.js

@@ -131,6 +131,7 @@ export default ModalComponent.extend({
         this.iconExtensions = ICON_EXTENSIONS;
         this.changedTiers = [];
         this.set('supportAddress', this.parseEmailAddress(this.settings.get('membersSupportAddress')));
+        this.set('openSection', 'signup-options');
     },
 
     didInsertElement() {

ghost/admin/app/styles/layouts/portal-settings.css

@@ -193,6 +193,7 @@
     flex-grow: 1;
     padding: 0;
     border-left: 1px solid var(--whitegrey);
+    background: linear-gradient(45deg, rgba(255,255,255,1) 0%, rgba(249,249,250,1) 100%);
 }
 
 .gh-portal-settings .modal-footer {