Fixing funcion in validations.py

[Rud1095]

Function validate user() works incorrectly, because it doesn't match dots and underscores and the start of the username.

validate_user("blue.kale", 3) # True

validate_user(".blue.kale", 3) # Currently True, should be False

validate_user("red_quinoa", 4) # True

validate_user("_red_quinoa", 4) # Currently True, should be False