base64.b64decode: always set validate=True

Notably verifymessage and decrypt(message) were silently ignoring trailing garbage
or inserted non-base64 characters present in signatures/ciphertext.
(both the CLI commands and in the GUI)
I think it is much cleaner and preferable to treat such signatures/ciphertext as invalid.

In fact I find it surprising that base64.b64decode(validate=False) is the default.
Perhaps we should create a helper function for it that set validate=True and use that.

electrum/daemon.py

@@ -221,7 +221,7 @@ class AuthenticatedServer(Logger):
         if basic != 'Basic':
             raise AuthenticationInvalidOrMissing('UnsupportedType')
         encoded = to_bytes(encoded, 'utf8')
-        credentials = to_string(b64decode(encoded), 'utf8')
+        credentials = to_string(b64decode(encoded, validate=True), 'utf8')
         username, _, password = credentials.partition(':')
         if not (constant_time_compare(username, self.rpc_user)
                 and constant_time_compare(password, self.rpc_password)):