PLT-3408 Add SiteURL to config.json (#3692)

* PLT-3408 Changed serverside code to get the service's URL from config.json

* PLT-3408 Changed most clientside code to use the SiteURL config setting instead of window.location

* PLT-3408 Changed default SiteURL to be autodetected

config/config.json

@@ -1,5 +1,6 @@
 {
     "ServiceSettings": {
+        "SiteURL": "",
         "ListenAddress": ":8065",
         "MaximumLoginAttempts": 10,
         "SegmentDeveloperKey": "",