add form_app (#463)

* add form_app

* revert unrelated changes

* use links in README

* remove unnecessary type annotation

* Remove GlobalKey for sign_in_http sample

* reword comment

* run package:copywriter

* use relative import

* fix quotes

* Add punctuation

* make sure setState() is used in form_widgets sample

* avoid first person pronoun

* store date state in parent widget in form_widgets.dart

experimental/form_app/web/manifest.json

@@ -0,0 +1,23 @@
+{
+    "name": "form_app",
+    "short_name": "form_app",
+    "start_url": ".",
+    "display": "standalone",
+    "background_color": "#0175C2",
+    "theme_color": "#0175C2",
+    "description": "A sample demonstrating different types of forms and best practices",
+    "orientation": "portrait-primary",
+    "prefer_related_applications": false,
+    "icons": [
+        {
+            "src": "icons/Icon-192.png",
+            "sizes": "192x192",
+            "type": "image/png"
+        },
+        {
+            "src": "icons/Icon-512.png",
+            "sizes": "512x512",
+            "type": "image/png"
+        }
+    ]
+}