Return a securely generated random string.
    With the a-z, A-Z, 0-9 character set:
    Length 12 is a 71-bit value. log_2((26+26+10)^12) =~ 71
    Length 44 is a 261-bit value. log_2((26+26+10)^44) = 261

def _get_random_string(length=44,
                       allowed_chars='abcdefghijklmnopqrstuvwxyz'
                       'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
                       secret_key=settings.secret_key_bytes()):
    """
    Return a securely generated random string.
    With the a-z, A-Z, 0-9 character set:
    Length 12 is a 71-bit value. log_2((26+26+10)^12) =~ 71
    Length 44 is a 261-bit value. log_2((26+26+10)^44) = 261
    """
    secret_key = _ensure_bytes(secret_key)
    _reseed_if_needed(using_sysrandom, secret_key)
    return ''.join(random.choice(allowed_chars) for i in range(length))