Add optional consent agreement datastream

[MarcusBarnes]

Often, one requires a consent form before publishing oral history interviews. The idea is to store either digitized versions of signed paper agreements or born-digital versions to that consent information can be bundled with the particular oral history object in question.

Based on the above, we have the following enhancement task: add an optional consent form datastream that will store an interview consent agreement with the oral history. This datastream should only be visible to administrators.

[timtomch]

@kimpham54 do you think the custom code you wrote for OCLS could be repurposed here? Similar use case.