Automatically remove other options when ALL selected/de-selected

[antonymilne]

Dropdowns with multi=True and checklists are a bit confusing when ALL is selected because it's easy to end up with the set of values [X, ALL] which works ok but is a bit awkward since then you need to manually de-select X.

Ideally:

• when current value = X and ALL is selected, X would be de-selected
• when current value = ALL and X is selected, ALL would be de-selected

It's not very important and maybe redundant if and when we finally replace our dropdown feature with something that works better with multi-select and all options in the future. But probably it's possible to do with a clientside callback.