S allius/issue387 (#388)

* add optional java script to fetch data regullary

* 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

app/src/.babel.cfg

@@ -1,2 +1,3 @@
 [python: **.py]
-[jinja2: web/templates/**.html]
+[jinja2: web/templates/**.html]
+[jinja2: web/templates/**.html.j2]