admin: change path behaviour for the billionth time

URLs are made with app.ExternalURI (now included in window.pages), which
itself should include Base. Also fixed the subfolder being removed from
the url after login.

models.go

@@ -485,5 +485,8 @@ type PagePaths struct {
 type PagePathsDTO struct {
 	PagePaths
 	// The subdirectory this bit of the app is hosted on (e.g. admin is usually on "/", myacc is usually on "/my/account")
-	Current string `json:"Current"`
+	Current     string `json:"Current"`
+	ExternalURI string `json:"ExternalURI"`
+	// The subdirectory the app is meant to be accessed from ("Reverse proxy subfolder")
+	TrueBase string `json:"TrueBase"`
 }