Allow events to mutate the DataBox

[wthrowe]

Proposed changes

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Further comments

[knelli2]

Can we come up with a better naming scheme than thing and actual_thing? If the original compute_item_tags isn't only compute items, maybe rename it to just item_tags? The class template pack should probably also change then since they aren't necessarily ComputeTags.

[wthrowe]

Since the new alias was only used in one place, I just inlined it.

I think renaming to item_tags would be confusing as there is already a tags_list that means something else.