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

[begedin]

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 code-corps/code-corps-api#1303.

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

[joshsmith]

Referencing my own comment on the API side, we may want to hold off on this for awhile. The temp-id is likely to become tid, indicating the churn in JSON API is high enough we likely don't want to tie ourselves to a draft that's not yet accepted.