Allow insertion and deletion at more arbitrary points in a proof

[NicMcPhee]

At the moment all you can do is add and delete at/from the end of the proof. It might make sense to have a delete button at every rule. I think the "right" thing to do there would be to delete the rule and the following expression, but I'm not 100% sure.

It also might make sense to allow insertion inside a proof. If, for example, a validate fails and you realize that you need to insert a step it would be nice to not have to copy/paste existing content down through the proof. I think the "right" thing here would be to have the + aligned with a rule, and it would insert a blank rule and expression above the existing rule.