Improve threads

[pm5]

Currently threads in Slack will appear in other bridged Slack channels as follows with a prefix [thread] to the message to indicate that it is in reply of a previous message:

The users need the following in this use case:

1. Know the full context of the message, i.e. all the previous messages.
2. Be able to reply in context, i.e. to send a reply that will appear in the thread in both local and remote Slack channels.

This bear some further investigations. Several things we could try to improve:

1. It is not clear which message is it replying to. At minimum a link to the previous message in the thread will help a lot in putting the it back to the context.
2. Of course it would be superb if remote threads can appear as ordinary threads in remote Slack channels.
3. It would be good if we can take other bridged services into consideration, such as IRC, Zulip, Discord. Perhaps a clear list of user behaviours regarding discussion threads is a first step. With that in hand we can see if it is possible to connect threaded discussions in ways that fit better into the services' users' habits, e.g. Slack threads could appear as <previous_speaker>: <message> on IRC.