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/crypto.py

@@ -473,7 +473,7 @@ def ecies_decrypt_message(
     *,
     magic: bytes = b'BIE1',
 ) -> bytes:
-    encrypted = base64.b64decode(encrypted)  # type: bytes
+    encrypted = base64.b64decode(encrypted, validate=True)  # type: bytes
     if len(encrypted) < 85:
         raise Exception('invalid ciphertext: length')
     magic_found = encrypted[:4]