AppModel: use fall back URL as preferred Home URL if in single domain

This should handle the most common case of a single Grist site with a
single server, removing the need for the user to set up APP_HOME_URL
in this case.

app/client/models/AppModel.ts

@@ -553,22 +553,28 @@ export function getConfiguredHomeUrl(): string {
 }
 
 /**
- * Get the home URL, using fallback if on admin page rather
- * than trusting back end configuration.
+ * Get the home URL, using fallback on the admin case and in the
+ * single-domain case case.
  */
 export function getPreferredHomeUrl(): string|undefined {
   const gristUrl = urlState().state.get();
-  if (gristUrl.adminPanel) {
+  const gristConfig: GristLoadConfig = (window as any).gristConfig;
+  if (gristUrl.adminPanel || gristConfig.serveSameOrigin) {
     // On the admin panel, we should not trust configuration much,
     // since we want the user to be able to access it to diagnose
     // problems with configuration. So we access the API via the
     // site we happen to be on rather than anything configured on
-    // the back end. Couldn't we just always do this? Maybe!
-    // It could require adjustments for calls that are meant
-    // to be site-neutral if the domain has an org encoded in it.
-    // But that's a small price to pay. Grist Labs uses a setup
-    // where api calls go to a dedicated domain distinct from all
-    // other sites, but there's no particular advantage to it.
+    // the back end.
+    //
+    // We can also do this in the common self-hosted case of a single
+    // domain, no orgs encoded in subdomains.
+    //
+    // Couldn't we just always do this? Maybe! It could require
+    // adjustments for calls that are meant to be site-neutral if the
+    // domain has an org encoded in it. But that's a small price to
+    // pay. Grist Labs uses a setup where api calls go to a dedicated
+    // domain distinct from all other sites, but there's no particular
+    // advantage to it.
     return getFallbackHomeUrl();
   }
   return getConfiguredHomeUrl();