mobile: Add about screen

mobile/lib/screens/about.dart

@@ -0,0 +1,40 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_markdown/flutter_markdown.dart';
+import 'package:url_launcher/url_launcher.dart' as url;
+
+class AboutScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final textTheme = theme.textTheme;
+
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('About'),
+      ),
+      body: FutureBuilder<String>(
+        future: rootBundle.loadString('assets/about.md'),
+        builder: (context, snapshot) {
+          if (snapshot.hasData) {
+            return Markdown(
+              data: snapshot.data,
+              styleSheet: MarkdownStyleSheet(
+                h1: textTheme.headline6.copyWith(
+                  height: 2.0,
+                ),
+                a: textTheme.bodyText1.copyWith(
+                  color: theme.accentColor,
+                  decoration: TextDecoration.underline,
+                ),
+              ),
+              onTapLink: (link) => url.launch(link),
+            );
+          } else {
+            return Container();
+          }
+        },
+      ),
+    );
+  }
+}