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/android/.gitignore

@@ -0,0 +1,11 @@
+gradle-wrapper.jar
+/.gradle
+/captures/
+/gradlew
+/gradlew.bat
+/local.properties
+GeneratedPluginRegistrant.java
+
+# Remember to never publicly share your keystore.
+# See https://flutter.dev/docs/deployment/android#reference-the-keystore-from-the-app
+key.properties