Counteroffer after offer is sent

[r4rdsn]

When offer is sent to counteragent, there are two buttons: "Accept" and "Decline", there is no way for counteragent to accept the offer with a different sum without contacting initiator directly.
There should be a third button "Counteroffer" which asks counteragent of a new sum and then sends it to initiator with the same three options.
I'm not sure how it would be integrated into the current initiator-counteragent data model, because they switch roles in this scenario. Maybe it could be implemented as a special case. In any case, this needs more thought.

[Kwaskoff]

Which help do you need?

[r4rdsn]

Which help do you need?

@Kwaskoff Just like it says in the issue, I'd like some contributions on the ways this can be integrated into the current data model, preferably so this could be readable in the code without a huge diff. I'd welcome any contributions on this issue, but if there won't be any by the time I get to it, I'll just have to do it the way I personally think would work best.
By the way please don't arbitrarily edit labels or move issues from "To do" list. I need these to sort my tasks.