Add a basic Purchase Thank you screen

lib/app.dart

@@ -5,6 +5,7 @@ import 'package:firebase_analytics/firebase_analytics.dart';
 import 'package:firebase_analytics/observer.dart';
 import 'package:flutter/material.dart';
 import 'package:gitjournal/screens/purchase_screen.dart';
+import 'package:gitjournal/screens/purchase_thankyou_screen.dart';
 import 'package:gitjournal/utils/logger.dart';
 import 'package:provider/provider.dart';
 import 'package:path/path.dart' as p;
@@ -137,6 +138,7 @@ class JournalApp extends StatelessWidget {
         '/onBoarding': (context) =>
             OnBoardingScreen(stateContainer.completeOnBoarding),
         '/purchase': (context) => PurchaseScreen(),
+        '/purchase_thank_you': (context) => PurchaseThankYouScreen(),
       },
       debugShowCheckedModeBanner: false,
       //debugShowMaterialGrid: true,

lib/screens/purchase_screen.dart

@@ -97,8 +97,7 @@ class PurchaseButton extends StatelessWidget {
             Settings.instance.proMode = true;
             Settings.instance.save();
 
-            // vHanda FIXME: Show some screen to indicate bought purchase?
-            Navigator.of(context).pop();
+            Navigator.of(context).popAndPushNamed('/purchase_thank_you');
             return;
           }
         } on PlatformException catch (e) {

lib/screens/purchase_thankyou_screen.dart

@@ -0,0 +1,37 @@
+import 'package:flutter/material.dart';
+
+class PurchaseThankYouScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    var theme = Theme.of(context);
+    var textTheme = theme.textTheme;
+    Widget w = Column(
+      children: <Widget>[
+        Text('Thank you', style: textTheme.display2),
+        Text(
+          "You're awesome for supporting GitJournal",
+          style: textTheme.display1,
+          textAlign: TextAlign.center,
+        ),
+        RaisedButton(
+          child: const Text("Back"),
+          color: theme.primaryColor,
+          onPressed: () {
+            Navigator.of(context).pop();
+          },
+          padding: const EdgeInsets.fromLTRB(64.0, 16.0, 64.0, 16.0),
+        )
+      ],
+      mainAxisAlignment: MainAxisAlignment.spaceAround,
+    );
+
+    return Container(
+      child: SafeArea(child: w),
+      color: theme.scaffoldBackgroundColor,
+      padding: const EdgeInsets.all(16.0),
+    );
+  }
+}
+
+// Ideas:
+// 1. Add a button to share about GitJournal on Twitter / Social Media over here