Handle edited messages (update_message events, 1/2)

[gnprice]

https://zulip.com/api/get-events#update_message

This issue is for the case where the message's contents were changed. (The case where the message changed stream and/or topic is #150.)

In particular these should be handled in the MessageListView view-model, so that the edits get reflected in a message list the user might be looking at.

For this issue, we'll:

• Add a new Event subclass for these events, similar to the others.
  • For this issue we can ignore new_stream_id, propagate_mode, and all the other properties that are documented at https://zulip.com/api/get-events#update_message as present only when the event moved messages. (We'll get to those later as part of Handle moved messages (update_message events, 2/2) #150.)
• Handle these events in PerAccountStore.handleEvent by calling a newly-added method on MessageListView, similar to the handling of MessageEvent.

[chrisbobbe]

Related issue:

• Handle deleted messages (delete_message events) #120

[gnprice]

Add a new Event subclass for these events, similar to the others.

This part is now done, as #212.

The remaining part of this issue is to handle the events.

[oxling]

👋 Hello! I'll take on the event handling.

[gnprice]

Sounds great! Marked on the issue metadata that you're taking it on.