Feature: Allow editing and deleting of previous messages

[sinus-x]

Currently, only the last message can be edited or deleted.

[sinus-x]

We should rework the message indexation, so we don't have mixed types as first item.

[ author | message1 (their or wormhole's) | message2 (wormhole) | ... ]

It's not dict, because we want to keep all user's messages until they expire.