feat: add how it's made link (#602)

functions/src/share/templates/footer.ts

@@ -10,7 +10,7 @@ export default `
     <ul>
       <li><a href="https://events.google.com/io/">Google I/O</a></li>
       <li><a href="https://flutter.dev/docs/codelabs">Codelab</a></li>
-      <li><a href="">How It's Made</a></li>
+      <li><a href="https://medium.com/flutter/i-o-photo-booth-powered-by-flutter-firebase-fc1381264e08">How It's Made</a></li>
       <li><a href="https://policies.google.com/terms">Terms of Service</a></li>
       <li><a href="https://policies.google.com/privacy">Privacy Policy</a></li>
     </ul>

lib/footer/widgets/footer_link.dart

@@ -109,7 +109,8 @@ class FooterHowItsMadeLink extends StatelessWidget {
     final l10n = context.l10n;
 
     return FooterLink(
-      link: '',
+      link:
+          'https://medium.com/flutter/i-o-photo-booth-powered-by-flutter-firebase-fc1381264e08',
       text: l10n.footerHowItsMadeLinkText,
     );
   }

test/app/widgets/footer_link_test.dart

@@ -175,7 +175,7 @@ void main() {
         expect(
           widget.link,
           equals(
-            '',
+            'https://medium.com/flutter/i-o-photo-booth-powered-by-flutter-firebase-fc1381264e08',
           ),
         );
       });