Currently, global IDs are generated using the full Base64 charset. This includes the characters + and /, which are not safe for displaying in URLs. The implementation the base64 utilities found [here](https://github.com/graphql-python/graphql-relay-py/blob/e9122d3a9754fc866ce5da64d3ccc3003a7b70d2/src/graphql_relay/utils/base64.py#L6) should use urlsafe_b64encode and urlsafe_b64decode respectively.
