Integration of pending users on disk so pending_users dictionary was eliminated.
All users are saved on disk (into file userdata.json) and have a flag, activated, set to true if the user is active or false if it's waiting for activation.
Changed function create_user into activate_user (which is now called with a PUT request).
Test of pending users integration.
Changed name function _manually_create_user into _manually_create_active_user which creates a user with flag active: True
Replaced POST action response from HTTP_OK to HTTP_CREATED because if an user is missing from disk (userdata.json) it's actually created with the POST action.
Added two functions:
_manually_create_inactive_user that creates a user with flag activated:False and saves it on disk;
_manually_remove_inactive_user that removes the user created with previous function from disk.
Modified method test_repeated_post.
This method test that every POST request of an inactive user returns HTTP_OK.
It means that user actually exists but it has the flag activated set to False.
Added method test_creation_inactive_user which for every new user POST request returns HTTP_CREATED.