Scope deployment lookup to project in DeploymentsController#show

What does this MR do and why?

Scope deployment lookup to project in DeploymentsController#show

A deployment belongs to both a project and an environment so either association could be used to fetch this record. However, because the IID is defined at the project level, looking up via project is a more efficient query as it can use the unique index on (project_id, iid).

References

#560329 (closed)

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