Allow to disable incoming video streams

[Elsensee]

Is your feature request related to a problem? Please describe.
When not in Wifi but on mobile data, data can be limited and apps should use that conservatively. Video calls with many participants can result in a lot of data usage.
Possibly related to #4795 ?

Describe the solution you'd like
I think this can be multi-step where each solution incrementally achieves a lot. In ascending order of dificulty to implement:

1. A toggle to disable all incoming video streams (and have that toggle automatically enabled when in data saving mode or battery saving mode or something like that maybe?)
2. A toggle per video stream (like on desktop) to disable the stream. I think on desktop this will only disable decoding and not receiving, which at least helps with battery, though?
3. A smart algorithm, which video streams the device should receive. Phones have a smaller display, so maybe not all video streams are necessary. Screenshare and speaker receive priority, though.

Of course this all only works with the HPB.

Describe alternatives you've considered
Asking all participants to disable their video or disabling video for the call.

Additional context
We are a small organisation that is currently in the process of setting up Nextcloud and "going all in" by switching our internal chat to Nextcloud Talk and our video conference tool as well. But during (video) calls, some of us are outside on their way and so only have a phone and mobile data. We currently use BigBlueButton which has the option to disable all incoming video and screenshares separately but it has a sub-par experience on mobile anyway. :/

[mahibi]

Thanks for the report, I will discuss possibilities for this with my colleagues.