message_update events disappear in private channels that are not newly created

[brakhane]

This time, no PR, as I'm not yet sure what the correct way to handle it is.

handle_message_update in State crashes when a private message is updated, because make_message tries to check message.channel.type. However, message.channel can return None if the channel is a private channel that we haven't seen yet (because it was created before a restart of the Bot, for example)

I think message.channel should, if it can't find a channel in the cache, call the Discord channel/{id} endpoint to get the necessary information

This is a pretty serious bug, BTW, because once it occurs, the bot basically stops handling any events and has to be restarted