Add ballot results to ballot serializer

[VirginiaDooley]

Closes #2172
Related to DemocracyClub/WhoCanIVoteFor#1685
The data I needed was already a part of results_bot process and the ResultSet model so I just needed to expose it the ballot serializer to be consumed by WCIVF.

To test, find a locked ballot with results entered or manually enter and lock a ballot.
Navigate to http://localhost:8000/api/next/ballots/local.st-albans.marshalswick-east-jersey-farm.by.2023-08-17/?format=json for example, and you should see data similar to the block below at the end of your request:

"results": { "num_turnout_reported": 300, "turnout_percentage": 7.5, "num_spoilt_ballots": 100, "source": "[google.com](http://google.com/)", "total_electorate": 4000 }

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[symroe]

Sorry, one more thing. The API docs don't look great:

[symroe]

I think you need to add ref_name = None # Tells swagger that this is always embedded to the MinimalResultSerializer (see example in CandidateResultSerializer)