Clearer `action` syntax

[lornajane]

Currently the action fields are:

• target
• description
• update
• remove

Having update and remove as siblings isn't great because if remove is "true" then update does nothing, they're quite exclusive. I'm sure there's a reason it was done like this, but I really can't tell what the reason is so I'm proposing that we instead do something like:

• target (as before)
• description (as before)
• type which can be "update" or "remove"
• update (as before, but only valid if type is update)

Feedback is very welcome on this idea!