Refactor users management

* Shard `src/data/user.rs` into two different files
  * One for user data structure (same file)
  * One for user manipulation (new file: `user_file_entity.rs`)
* Isolate password hashing and verification

src/data/mod.rs

@@ -15,4 +15,5 @@ pub mod remote_ip;
 pub mod session_identity;
 pub mod totp_key;
 pub mod user;
+pub mod users_file_entity;
 pub mod webauthn_manager;