Add support for searching scrollback for specific posts/threads

[hloeung]

This reduces the needs for searching through IRC client logs for specific post/threads by Mattermost IDs. Instead, we can use the scrollback feature, e.g.:

# Only show the first 5 posts in thread.
/msg mattermost scrollback 1n4gsc6g638q9y53drezgf44gy 5

# Show all posts within the returned thread.
/msg mattermost scrollback 1n4gsc6g638q9y53drezgf44gy 0

[hloeung]

Added workaround for mattermost/mattermost#23081

[42wim]

matterclient PR has been merged

[42wim]

vendored file needs to be removed also :)

[hloeung]

vendored file needs to be removed also :)

Oh ok, reverted the changes to the vendored file

[hloeung]

Cleaning it up cherry-picking the relevant commits into master

[hloeung]

Okay, now this is merge-able.