Enable a reasignment of last peg / unsending last peg

[arthurs]

Occasionally people assign pegs to the wrong person, would be nice to have a way to fix pegs that have been given. though should be limits, options i can think of

• only undo the last peg you sent (no time limit?)
• have x time to undo a send peg? how do you do multi pegs?
• admin only feature, save database edits

a possible implementation could be responding the the PM from pockybot. could investigate how the reply to thread acts in relation to webhooks

[jimcullenaus]

could investigate how the reply to thread acts in relation to webhooks

This is already under investigation in #69 for a different use case. Could be used here once that is done.