Pending user integration

Pending users are now saved on disk.
Every user saved on disk (into userdata.json file) has a flag called "activated" which is True when a PUT request with an equivalent activation code is made, and False when a POST request is made.
Therefore, some refactoring was made regarding "create_user" function which becomes  "activate_user" and now activates an already existing but inactive user.
On the other hand, the tests were also improved to test new functionalities like "test_creation_inactive_user", "test_repeated_post".
il repr non dovrebbe essere necessario.. puoi verificare?