Load step.node when querying for LogSources in JobDetailsAPIView.get
Load step.node when querying for LogSources in JobDetailsAPIView.get

Summary:
Serialized LogSource includes JobStep which includes Node, so this joinedload avoids len(nodes)
queries (often ~50 in prod).

Test Plan: Manual

Reviewers: anupc

Reviewed By: anupc

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D192828

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