fix: Let calling message update the event timestamp when before a deleted message

[atomrc]

STR:

• start a call in a group (pick up or not, is doesn't affect the result)
• send a text message
• delete that text message
• send another text message
  -> the last text message doesn't show up

The problem

The problem is that the conversation timestamp gets updated backward when deleting the last event of the conversation.
But because call events are not supposed to affect the timestamp, in this case we try to update the timestamp to the timestamp of a message that is not supposed to update it.

That means that the conversation last event timestamp will be in the future compared to the last loaded event (which typically means that the conversation is scrolled up and we are not looking at the last message of the conversation).
In this case the webapp doesn't append new messages at the end of the message list.

Notes for the future

This timestamp computation is pretty messy and would need a clean rethink at some point