Modify Conversation serializer to handle includes based on JSON API sideposting draft

# Problem

From https://github.com/json-api/json-api/pull/1197/files?short_path=571f6ce#diff-571f6cee9a492e0e177801c21a4b2b2a

When creating a `conversation` alongside a `message`, we need to generate a `tempId` for that conversation. The `tempId` needs to appear as `temp-id` both in the json['data']['relationship'] identifier as well as the `json['included']` section and needs to be posted to the API.

## References

Needs to be implemented together with #1303 https://github.com/code-corps/code-corps-api/issues/1303.

The code this needs to modify is in #1597, so this should be considered blocked until that is merged.