add a setup screen to create the first admin user

If you prefer to auto-create the first admin you can enable the
"create_default_admin" configuration key and SFTPGo will work as before.

You can also create the first admin by loading initial data: now you can
set both username and password, before you could only change the password

common/protocol_test.go

@@ -60,6 +60,7 @@ func TestMain(m *testing.M) {
 	logFilePath := filepath.Join(configDir, "common_test.log")
 	logger.InitLogger(logFilePath, 5, 1, 28, false, zerolog.DebugLevel)
 
+	os.Setenv("SFTPGO_DATA_PROVIDER__CREATE_DEFAULT_ADMIN", "1")
 	err := config.LoadConfig(configDir, "")
 	if err != nil {
 		logger.ErrorToConsole("error loading configuration: %v", err)