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/tests/test_web_route.py

@@ -61,3 +61,13 @@ async def test_manifest():
     response = await client.get('/site.webmanifest')
     assert response.status_code == 200
     assert response.mimetype == 'application/manifest+json'
+
+@pytest.mark.asyncio
+async def test_data_fetch():
+    """Test the healthy route."""
+    client = app.test_client()
+    response = await client.get('/data-fetch')
+    assert response.status_code == 200
+
+    response = await client.get('/data-fetch')
+    assert response.status_code == 200