Added new basic Loading page for data init

We want to show some kind of loading state till we fetch initial site or member data to load the relevant page

ghost/portal/src/components/LoadingPage.js

@@ -0,0 +1,13 @@
+const React = require('react');
+
+export default class LoadingPage extends React.Component {
+    render() {
+        return (
+            <div style={{display: 'flex', flexDirection: 'column', color: '#313131'}}>
+                <div style={{paddingLeft: '16px', paddingRight: '16px', paddingTop: '12px'}}>
+                    Loading...
+                </div>
+            </div>
+        );
+    }
+}

ghost/portal/src/components/PopupMenuComponent.js

@@ -3,6 +3,7 @@ import SignupPage from './SignupPage';
 import SigninPage from './SigninPage';
 import SignedInPage from './SignedInPage';
 import MagicLinkPage from './MagicLinkPage';
+import LoadingPage from './LoadingPage';
 
 const React = require('react');
 const PropTypes = require('prop-types');
@@ -43,6 +44,11 @@ const Styles = {
             width: '400px',
             minHeight: '130px',
             maxHeight: '130px'
+        },
+        loading: {
+            width: '250px',
+            minHeight: '130px',
+            maxHeight: '130px'
         }
     },
     popup: {
@@ -75,7 +81,8 @@ const Pages = {
     signin: SigninPage,
     signup: SignupPage,
     signedin: SignedInPage,
-    magiclink: MagicLinkPage
+    magiclink: MagicLinkPage,
+    loading: LoadingPage
 };
 
 export default class PopupMenuComponent extends React.Component {