Allow curators to effectively 'clear' the annotation extension relation field

[vanaukenk]

If a curator selects an annotation extension relation and then decides they don't want to make an extension, there is no way to clear the relation field.

We need to either always have a blank field available, or otherwise have a way to clear that field.

[vanaukenk]

From 2024-06-06 workbenches call:

We will add a clear 'x' button for curators to click if they want to clear a value in any field.