add web_dashboard API (#333)

* add web_dashboard sample

* add docs

* address code review comments

* restructure web_dashboard

* set up provider and mock service

* add copyright headers, use relative imports

* add API class, add tests

* fmt

* rename services -> API, remove data library

* use new API in app

* add stream to items api

* convert from StreamBuilder to StreamProvider

* add subscription to Entry API

* rename API classes

* address comments

* Update README.md

* update README

* remove routing_demo

experimental/web_dashboard/README.md

@@ -0,0 +1,20 @@
+# web_dashboard
+
+(In progress) A dashboard app for the web, built with Flutter.
+
+This app is web-first, and isn't guaranteed to run on iOS, Android or desktop
+platforms.
+
+## Running
+
+Normal mode (DDC):
+
+```
+flutter run -d chrome
+```
+
+Skia / CanvasKit mode:
+
+```
+flutter run -d chrome --release --dart-define=FLUTTER_WEB_USE_SKIA=true
+```