[OS-906] Fix deprecated HTML Imports (#432)

This commit fix HTML Imports deprecated syntax, install x-tag through bower and apply the package directly. It does not to change any functionality.

Author: lauraenria

bower.json

@@ -17,7 +17,8 @@
   ],
   "dependencies": {
     "web-components": "[email protected]:KanoComputing/web-components",
-    "iron-image": "PolymerElements/iron-image#^1.2.5"
+    "iron-image": "PolymerElements/iron-image#^1.2.5",
+    "xtag": "^0.1.1"
   },
   "resolutions": {
     "iron-image": "^2.1.2",