Fixed initial role selection and layout shift in <GhRoleSelection>

no issue

- initial role was always being set to Contributor due to incorrect argument being used when checking if the default role should be applied
- there was considerable layout shift of the `<GhRoleSelection>` container after role loading was finished, quick-fix applied by specifying a width and height to the loading spinner container that matches the final width/height

ghost/admin/app/components/gh-role-selection.hbs

@@ -1,6 +1,9 @@
 <div class="gh-roles-container" {{did-insert (perform this.fetchRolesTask)}}>
     {{#if this.fetchRolesTask.isRunning}}
-        <GhLoadingSpinner />
+        {{!-- width and height are needed to avoid layout shift after roles have loaded --}}
+        <div style="width: 486px; height: 268px">
+            <GhLoadingSpinner />
+        </div>
     {{else}}
     <div class="gh-radio {{if (eq @selected.name "Contributor") "active"}}" {{on "click" (fn this.setRole "Contributor")}} data-test-option="Contributor">
         <div class="gh-radio-button"></div>

ghost/admin/app/components/gh-role-selection.js

@@ -28,7 +28,7 @@ export default class GhRoleSelectionComponent extends Component {
 
         this.roles = roles;
 
-        if (!this.args.role && defaultRole) {
+        if (!this.args.selected && defaultRole) {
             this.args.setRole(defaultRole);
         }
     }