Add weback-dev-server with hot/hmr support

* adds `npm start` / `yarn start` script
* starts a webpack-dev-server using the dev config, served on :3333
* hot reloading (HMR) for react/styles, not working for angular code
* new entry `dev.ts` for dynamic imports of CSS theme (ExtractText does
not work with HMR)
* TS loader pipeline moved out of common to add HMR for react
* applied `hot()` to some react containers (that's their new default
 export, named exports remains for testing)
* added sections to README
* updated yarn.lock

README.md

@@ -39,12 +39,21 @@ go run build.go build
 
 For this you need nodejs (v.6+).
 
+To build the assets, rebuild on file change, and serve them by Grafana's webserver (http://localhost:3000):
 ```bash
 npm install -g yarn
 yarn install --pure-lockfile
 npm run watch
 ```
 
+Build the assets, rebuild on file change with Hot Module Replacement (HMR), and serve them by webpack-dev-server (http://localhost:3333):
+```bash
+yarn start
+# OR set a theme
+env GRAFANA_THEME=light yarn start
+```
+Note: HMR for Angular is not supported. If you edit files in the Angular part of the app, the whole page will reload.
+
 Run tests 
 ```bash
 npm run jest
@@ -55,6 +64,8 @@ Run karma tests
 npm run karma
 ```
 
+Run
+
 ### Recompile backend on source change
 
 To rebuild on source change.