MM-9983 Requiring SiteURL to be set. (#8769)

* Requiring SiteURL to be set.

* Modifying to make tests pass.

* Fixing test.

config/default.json

@@ -1,6 +1,6 @@
 {
     "ServiceSettings": {
-        "SiteURL": "http://localhost:8065",
+        "SiteURL": "",
         "WebsocketURL": "",
         "LicenseFileLocation": "",
         "ListenAddress": ":8065",