Add a POC OAuth client implementation

I've registered GitJournal as an app on Github and tried to get the
access_token within the app. This seems to work. Though, I need to
provide the client secret as well. It's quite sad that GitHub does not
even support the Implicit Grant auth type.

Credit: https://stackoverflow.com/questions/46196240/oauth2-flow-in-flutter-app

android/app/src/main/AndroidManifest.xml

@@ -35,5 +35,17 @@
                 <category android:name="android.intent.category.LAUNCHER"/>
             </intent-filter>
         </activity>
+
+        <activity
+            android:name=".UriReceiverActivity"
+            android:parentActivityName=".MainActivity">
+            <intent-filter>
+                <action android:name="android.intent.action.VIEW" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <category android:name="android.intent.category.BROWSABLE" />
+                <data android:scheme="gitjournal" android:host="login.oauth2" />
+                <!-- Triggering URI would be gitjournal://login.oauth2 -->
+            </intent-filter>
+        </activity>
     </application>
 </manifest>

android/app/src/main/java/io/gitjournal/gitjournal/MainActivity.java

@@ -22,13 +22,15 @@ import io.flutter.util.PathUtils;
 
 public class MainActivity extends FlutterActivity implements MethodCallHandler {
     private static final String CHANNEL_NAME = "gitjournal.io/git";
+    static MethodChannel channel;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         GeneratedPluginRegistrant.registerWith(this);
 
-        new MethodChannel(getFlutterView(), CHANNEL_NAME).setMethodCallHandler(this);
+        channel = new MethodChannel(getFlutterView(), CHANNEL_NAME);
+        channel.setMethodCallHandler(this);
     }
 
     @Override

android/app/src/main/java/io/gitjournal/gitjournal/UriReceiverActivity.java

@@ -0,0 +1,35 @@
+package io.gitjournal.gitjournal;
+
+import android.os.Bundle;
+import android.util.Log;
+import android.app.Activity;
+import android.net.*;
+
+import android.content.Intent;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class UriReceiverActivity extends Activity {
+    @Override
+    protected void onCreate(Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+        Uri data = getIntent().getData();
+
+        Map<String, Object> map = new HashMap<>();
+        map.put("URL", data.toString());
+
+        MainActivity.channel.invokeMethod("onURL", map);
+
+        // Now that all data has been sent back to Dart-land, we should re-open the Flutter
+        // activity. Due to the manifest-setting of the MainActivity ("singleTop), only a single
+        // instance will exist, popping the old one back up and destroying the preceding
+        // activities on the backstack, such as the custom tab.
+        // Flags taken from how the AppAuth-library accomplishes the same thing
+        Intent mainIntent = new Intent(this, MainActivity.class);
+        mainIntent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
+        startActivity(mainIntent);
+        finish();
+    }
+
+}

lib/oauthapp.dart

@@ -0,0 +1,51 @@
+import 'package:flutter/material.dart';
+import 'package:journal/apis/git.dart';
+import 'package:flutter/services.dart';
+import 'package:url_launcher/url_launcher.dart';
+
+const _platform = const MethodChannel('gitjournal.io/git');
+
+// Actual message handler:
+Future _handleMessages(MethodCall call) async {
+  switch (call.method) {
+    case "onURL":
+      print("Call onURL" + call.arguments.toString());
+    // Do something nice using call.arguments["URL"]
+  }
+}
+
+class OAuthApp extends StatefulWidget {
+  @override
+  OAuthAppState createState() {
+    return new OAuthAppState();
+  }
+}
+
+class OAuthAppState extends State<OAuthApp> {
+  void initState() {
+    super.initState();
+    _platform.setMethodCallHandler(_handleMessages);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return new MaterialApp(
+      title: 'OAuth App',
+      home: Scaffold(
+        appBar: AppBar(
+          title: Text('OAuth Test'),
+        ),
+        body: Column(children: <Widget>[
+          RaisedButton(
+            child: Text("Open OAuth URL"),
+            onPressed: () {
+              var url =
+                  "https://github.com/login/oauth/authorize?client_id=aa3072cbfb02b1db14ed&scope=repo";
+              launch(url);
+            },
+          ),
+        ]),
+      ),
+    );
+  }
+}