After acking or resolving, show all open incidents

[technicalpickles]

If someone is paged, and they are in chat, it's usually easiest to just /pager ack from chat. The only output is a list of incident numbers acknowledged. So, usually there's a follow up /pager sup to see open list.

It'd be cool to include this by default. Or, if not that, then at least have a short summary of what's triggered vs acknowledged.

cc @jnewland @jssjr