refactor frontend to use WP REST API instead of WP AJAX

Query new ticks using GET request to the REST API.
Also we now trigger one request per ticker, if more than one is
available on the same page.

HTML markup (list element, content container) is generated client side,
s.t. we can work on the generic response data model

.eslintrc.json

@@ -4,7 +4,7 @@
     "browser": true
   },
   "globals": {
-    "sclivetickerAjax": "readonly",
+    "scliveticker": "readonly",
     "wp": "readonly"
   },
   "extends": [