Bumped Portal to 2.28

- this bumps the version to 2.28 in Ghost so we can use the latest and
  greatest

ghost/core/core/shared/config/defaults.json

@@ -176,7 +176,7 @@
     },
     "portal": {
         "url": "https://cdn.jsdelivr.net/ghost/portal@~{version}/umd/portal.min.js",
-        "version": "2.27"
+        "version": "2.28"
     },
     "sodoSearch": {
         "url": "https://cdn.jsdelivr.net/ghost/sodo-search@~{version}/umd/sodo-search.min.js",