MM-11241: fix context site url header (#9079)

* fix context's SiteURLHeader to use the subpath

This fixes various commands such as /open and /groupmsg, as well as
OAuth related redirects.

* MM-11246: also leverage GetSiteHeader() in completeSaml

web/handlers.go

@@ -68,7 +68,9 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	c.SetSiteURLHeader(app.GetProtocol(r) + "://" + r.Host)
+	subpath, _ := utils.GetSubpathFromConfig(c.App.Config())
+	siteURLHeader := app.GetProtocol(r) + "://" + r.Host + subpath
+	c.SetSiteURLHeader(siteURLHeader)
 
 	w.Header().Set(model.HEADER_REQUEST_ID, c.RequestId)
 	w.Header().Set(model.HEADER_VERSION_ID, fmt.Sprintf("%v.%v.%v.%v", model.CurrentVersion, model.BuildNumber, c.App.ClientConfigHash(), c.App.License() != nil))