A `!tunnel` command for ease of redirecting conversations

[Matiiss]

This issue is mostly for keeping track of what was proposed (by me) in #community-meta

The first iteration of the implementation (what this issue is for) would target the following idea by @jb3:

I'm envisaging it being fairly lightweight:

➡️ Conversation continued in ⁠#ot0-yes-do-as-i-sayǃ

↩️ Conversation tunneled by @joe from ⁠#python-discussion

(where those are message links, not channel links)

Original proposal:

I have a proposal for a !tunnel feature for Lancebot

Suppose a discussion in #python-discussion is starting to veer off topic, so you'd like to move to one of the ot channels, but there isn't an easy way to cross-link the now-separated strands of conversation for some future reference (somebody either trying to find the beginning of the conversation going from ot backwards, or someone trying to find where the conversation went going from python-discussion forward). But this feature would create the bi-directional "portal" links, a link to a message in the ot channel in python-discussion and that message in the ot channel containing a link back to the tunnel-entry message that points back to itself.

Implementation-wise, !tunnel would by default pick one of the ot channels with the oldest recent message and use that, but otherwise it'd be possible to also specify a channel you want to tunnel to. The source would naturally always be the current channel the command is invoked in (though I guess, it could support a "from" keyword-only argument for the the source too, I don't see it being used that often? I guess if you wanted to invoke this command in one of the ot channels to bring a conversation from say python-discussion to there?).

So then, after the source and destination channels are sorted out, it would post a message in the source channel, post a message in the destination channel with a link to the message in the source channel, and then edit the message in the source channel with the link to the message in the destination channel.
Mildly important, but the messages should contain text that is portal-technology-themed.

This might help reduce friction in moving conversations to another channel.

See also: #dev-contrib