Chat alternative setup

Crowdcast (#2) had a chat which is a requirement but with cons. Jitsi (if to go with it as the video conference and casting) also has a chat, but it is IMHO also too simplistic and not persistent (unless keeping channel open). I think we better integrate with some other chat which then could be used not only for video chat or casting sessions, but throughout the entire platform. Somewhat akin to disqus which could provide commenting for each web page on the website. Then we could have consistent with desired features (archival, persistency, `@references`, permalinks) for any element in the platform and/or full sub hierarchy (eg see chats going on across all posters in a section).

- mattermost?
- could be integrated: https://docs.mattermost.com/integrations/embedding.html .
- there is jitsi plugin for mattermost now: https://mattermost.com/blog/mattermost-and-jitsi/
- @r03ert0 pointed to https://github.com/42wim/matterbridge which might be handy
- https://rocket.chat/ !!! - mini-conf already [integrates it](https://github.com/Mini-Conf/Mini-Conf/pull/52/files) ! (although it seems to fail to load for me on their [demo website](http://www.mini-conf.org/chat.html)). [no groupping of chat rooms yet](https://github.com/RocketChat/feature-requests/issues/637) but otherwise looks very fitting
- some other (more light weight) solution?