Switch to standard base64

The exact format is unimportant, what matters is that the nonce contains
enough entropy. Crypt64 was originally chosen because it was used elsewhere.
Raw bytes would also work, but would be less readable if inspected manually.

src/server/user_handler.cpp

@@ -68,6 +68,6 @@ std::string user_handler::create_secure_nonce()
 		throw RAND_bytes_exception();
 	}
 
-	return crypt64::encode({buf.data(), buf.size()});
+	return base64::encode({buf.data(), buf.size()});
 }