cmd/syncthing, gui: Settings dialog upgrades/reporting for candidates (fixes #4216) (#5457)

This adds booleans to the /system/version response to advice the GUI
whether the running version is a candidate release or not. (We could
parse it from the version string, but why duplicate the logic.)

Additionally the settings dialog locks down the upgrade and usage
reporting options on candidate releases. This matches the current
behavior, it just makes it obvious what actually *can* be chosen.

cmd/syncthing/gui.go

@@ -609,12 +609,15 @@ func (s *apiService) getJSMetadata(w http.ResponseWriter, r *http.Request) {
 }
 
 func (s *apiService) getSystemVersion(w http.ResponseWriter, r *http.Request) {
-	sendJSON(w, map[string]string{
+	sendJSON(w, map[string]interface{}{
 		"version":     Version,
 		"codename":    Codename,
 		"longVersion": LongVersion,
 		"os":          runtime.GOOS,
 		"arch":        runtime.GOARCH,
+		"isBeta":      IsBeta,
+		"isCandidate": IsCandidate,
+		"isRelease":   IsRelease,
 	})
 }