Add Subfield 3 to 591 index

[ruthtillman]

The cataloger-generated 591s may contain a subfield 3 (though this shouldn't be the whole record). Can we add this to our index? I know 591s are a bit complex since we have the structured system-generated ones too. The data would be up front as the field means "materials specified" so normally that would be something like |3Special Collections Copy: |aIn Sharebox 111.

Not a rush but a recent cataloger error exposed this as something we could support on the offchance it was ever done as: |3Special Collections Copy: In Sharebox 111