Allow jumping to older messages from replies.

[tandr3w]

Currently, when jumping to a previous message for a reply, if the original message isn't loaded, it gives an error like "ref does not exist". I think it would be useful if it was possible to jump to a message even if it hasn't been fetched yet. If a full-text search was implemented, this would also be useful to allow jumping to messages. Something like:

1. Fetch only the original message you want to jump to and some messages around it, then scroll to that message.
2. When scrolling down, fetch the messages below, similarly to how messages are currently fetched, just in a different direction.

I think that one possible issue with this implementation is that it's possible for the original messages to be fetched but not the edited versions if messages are fetched while scrolling down (this doesn't happen while scrolling up since it's guaranteed for edits to be later than the original message). Can we do anything about this?