Component: Video casting

Largely inspired by overview of pros/cons of crowdcast (see #2) for needs to fulfill. Here concentrating on video casting aspect ATM.

Desired features
- scale to few thousands of recepients (dedicated CDN setup, ideally with auto-scaling etc)
- integrate with interactive features (Q&A #11 , chat #4 )
- allow for different sources of 'audio video' input:
- video file (pre-recorded video)
- mix-in of (possibly multiple) presenters (talk, panel discussions etc) with support for screen sharing. So pretty much from #12
- recording and export to other casing platforms (e.g. youtube), most likely also just need to be done for #12

References:
- jitsi + hack https://community.jitsi.org/t/how-do-i-change-youtube-live-stream-to-another-rtmp-server-url/24817/46 to cast to other than YouTube rtmp casting server/service
- Custom setup of debconf: https://debconf-video-team.pages.debian.net/docs/index.html and [USB installer](https://debconf-video-team.pages.debian.net/ansible/usb_install/usb_quick_start.html) setup for video mixing etc . Distribution of content via a bunch of (cheap!) Digital Ocean VMs. Some stats for debconf20 could be found on 8th minute slide within [closing ceremony video](https://saimei.ftp.acc.umu.se/pub/debian-meetings/2020/DebConf20/94-closing-session-so-long-and-thanks-for-all-the-debs.webm): 1481 unique watchers, 251 simultaneous (I guess max)
- FOSDEM! Similarly to debconf - everything is based on open tech, was always a "hybrid" ([description of video capturing setups](https://archive.fosdem.org/2020/schedule/event/videobox/attachments/slides/4201/export/events/attachments/videobox/slides/4201/fosdem_videobox_202002021338_libv.pdf)), **many** parallel tracks, videos are [publicly archived](https://video.fosdem.org/). It is larger than DebConf, (crazy crazy and good, I even managed to attend FOSDEM 2020). Setup details: https://github.com/FOSDEM/infrastructure/tree/master/ansible .Told by Debian-video people that it is similar to Debconf.

Notes about debconf setup scalability

dump from IRC question

```
13:05 yoh_: re debconf2020 video casting: So about 1.4k visitors, and ~250 max watchers
at any given moment. Do you think the setup would scale (given more CDN
servers deployed) seamlessly to 1-3k simultaneous watchers?
13:08 pollo: yoh_: yes, the load on the streaming frontends wasn't that high
13:08 olasd: we've hit some surprising nginx-rtmp behaviors (mirrored streams dropping
off and not recovering until a nginx restart) that would need
investigation before adding more CDN servers; but in general, yeah,
several 1000s simultaneous watchers shouldn't be a problem
13:09 olasd: CCC and fosdem use a similar setup with these kinds of numbers without an
issue, AFAIK
```