Adding utilities to open urls to emacs-slack

Some years ago I opened a fork of the amazing https://github.com/yuya373/emacs-slack because useful PRs would not flow into the author's repository. Interestingly, the whole project got deprecated recently.

Is emacs-slack dead? Nope, thanks to contributors like isamert!

Some of the issues looked interesting to me, so I spent some of my (very little) time to play with my fork.

Mainly I wanted ways to jump back and forward from the slack app (which allows me to take calls, called huddles in slack). This way I can easily keep using emacs-slack to add notifications of messages in my Org Agenda and pop to the app when needed.

Also managed to solve a very nasty little error that made me lose quite some work: sometimes slack thread buffers would close on me. This was due to the websocket (with which channel info get updated) killing the buffer.

Overall quite happy of the results. There are still many things I don't know about this repository (particularly on the websocket side, which apparently easily fails if too much data is pulled from slack), but curious to repair and improve it as I need it.

Happy chatting!