Download safeguarding files from Google Drive (optionally)

[istride]

During the 'pull' phase, the safeguarding XLSX workbook can be downloaded via the Google Drive API. The workbook must be uploaded to Drive as an XLSX file. Once uploaded, grab the Drive file id (from the URL) and add it to the 'sources' configuration for safeguarding.

"safeguarding": {
    "format": "safeguarding",
    "sources" : [
        {
            "key": "ara",
            "location": "1M9CgURmS433MuF1lwc4fMFYsmCJAUhE3"
        }
    ]
}

Note the key location, instead of path. Both path and location are usable, but only location will trigger a download from Drive. The path key should be considered deprecated and will be removed in a future release.

Resolves #143

[geoo89]

Looks good to me. I'll leave it to Ed to test this.

As per my comment in the issue, I thought this could be an opportunity to change the format of the safeguarding sheets to something more sane that can be handled by the standard source format, but this works too.