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.

先查看 #1597 中的 serializer 變更，此 issue 將其列為先決條件，並閱讀 #1303 了解相關實作。驗證 JSON API 的 sideposting 草稿，並追蹤 conversation 與 message 一起建立的流程。完成的標準是：產生的 tempId 在 relationship identifier 和 json['included'] 中都以 temp-id 的形式輸出，然後送出至 API。

由索引模型根據 Issue 內容生成。