[Feature request] Add explanation of why user cannot relink/remap a media record

[themerekat]

When re-linking a media record is disabled (i.e., the catalog number matches the media record name), show a message that tells the user WHY they cannot remap the image to another record.

Right now, the option just doesn't show up, which leads to much confusion.

[ljwalker]

Lindsay can think of at least two Help Desk tickets where this has been an issue in the last month.