Automatically create admin on first start

src/constants.rs

@@ -0,0 +1,6 @@
+/// File in storage containing users list
+pub const USERS_LIST_FILE: &str = "users.json";
+
+/// Default built-in credentials
+pub const DEFAULT_ADMIN_USERNAME: &str = "admin";
+pub const DEFAULT_ADMIN_PASSWORD: &str = "admin";