rails: source file URIs should be unique (#92)
rails: source file URIs should be unique (#92)

We want to protect against the same source file being recorded twice.
With the uniqueness constraint, trying to POST a new source file with
the same URI as an existing source file results in:

    {
        "errors": [
            {
                "code": "105",
                "detail": "day is not allowed.",
                "status": "400",
                "title": "Param not allowed"
            }
        ]
    }

Closes #92.

HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence,HackBrexit/MinistersUnderTheInfluence