S allius/issue387 (#389)

* add optional java script to fetch data regulary


* change file extension to `html.j2` for templates

* fix route for fetch data

- for running in iframes (e.g. HA ingress) we must
  use relative path in the URLs

* increase test coverage

* remove unused statements

* update favicon.svg

app/src/web/static/images/site.webmanifest

@@ -1,6 +1,6 @@
 {
   "name": "TSUN-Proxy",
-  "short_name": "TsunProxy",
+  "short_name": "Proxy",
   "icons": [
     {
       "src": "/web-app-manifest-192x192.png",