Software: Loader that loads until LSL stream is resolved

[mlej8]

This is a full-stack feature.

Once the OpenBCI process is started in the frontend, we need to confirm that a stream has been resolved by this OpenBCI process. Only then, we can start watching videos. Hence, we might want to add a variable after the resolve_stream call in stream.py that gets toggled to indicate that a stream has been resolved and that we are able to receive OpenBCI data from the OpenBCI GUI. The frontend can then check on this variable to see whether a stream has been resolved and let the user go to the first video.

The front end should block at the video selection page until the stream is resolved on the backend.

@liuyyn @cherryman @archarbar @superkaiba @ding-ma @BaranchukM @joeyxoay feel free to work together on this task.