Allow to bulk-remove expiry date and context from selected nodes

[emilyy-dev]

When selecting nodes, it only lets you update (or add in the case of context) the current value for those fields, but not remove them, would be nice if you could remove/delete the current state of them :)

[Turbotailz]

The expiry date was maybe an oversight. I could have sworn I made it possible to remove the contexts though?