Add demo mode restrictions to user operations.

Added functionalities to restrict certain user operations like update, password change, and deletion in demo mode. This is done to prevent demo users from modifying crucial data. Helper methods are created for standard re-usable checks. Also, Blade directive is added for frontend UI demo checks.

.env.example

@@ -4,6 +4,10 @@ APP_KEY=
 APP_DEBUG=true
 APP_URL=http://localhost
 
+APP_DEMO_MODE=false
+APP_DEMO_EMAIL=
+APP_DEMO_PASSWORD=
+
 APP_DEFAULT_USER_TIMEZONE=UTC
 # Valid languages: ru | en
 APP_DEFAULT_LOCALE=ru