Use 128 bits of entropy instead of only 32 in csp/ub.py

Remote login tokens are generated from only 4 bytes of randomness (32 bits = ~4
billion possibilities, 8 hex characters). The /ajax/verify_token endpoint at
remotelogin.py:98 has no rate limiting. The token is valid for 10 minutes.

At even modest request rates (10,000 req/sec), an attacker can test ~6 million
tokens during the 10-minute window , which isn't enough to exhaust the full
space, sure, but combined with multiple concurrent login sessions (each
generating a new token), or if the attacker can trigger the victim to initiate
remote login, the attack becomes more feasible. Compare with the Kobo auth
token which uses urandom(16) (128 bits).

cps/ub.py

@@ -537,7 +537,7 @@ class RemoteAuthToken(Base):
 
     def __init__(self):
         super().__init__()
-        self.auth_token = (hexlify(os.urandom(4))).decode('utf-8')
+        self.auth_token = (hexlify(os.urandom(16))).decode('utf-8')
         self.expiration = datetime.now() + timedelta(minutes=10)  # 10 min from now
 
     def __repr__(self):