start_stream_stream_slot_does_not_exist issue

[iccanobif]

Steps to reproduce:

• user A clicks on "start stream" but keeps the popup open
• user B starts a stream in another slot
• user A completes the popup wizard
• user A is met with a start_stream_stream_slot_does_not_exist error message

This happens because updateCurrentRoomStreams() clears this.streamSlotIdInWhichIWantToStream for everyone and restores it to its original value only if their stream is active... This logic was meant to handle cases in which a stream is forcefully stopped by the server (usually during the daily restart) and i guess it's probably best to split the two concepts: "slot id in which i want to stream" vs "slot id in which I AM streaming". Careful that at the moment streamSlotIdInWhichIWantToStream is used here and there as a way to understand if the user is streaming or not (I'm trying to encapsulate this logic in the isStreaming() function so that once we have a better variable to use we can change just that part).