Add toggle to show and hide video from other participants

Before a user could disable her own video, but she had no way to disable
the videos from other participants. This could be needed, for example,
to alleviate the load on low-end systems, as in that case playing the
video from remote participants could be too much for the system. Now a
toggle is provided to manually show or hide the video of each remote
participant if needed.

The toggle is shown only when the remote participant is sending video;
if the remote participant has disabled her own video (or does not have a
camera) the toggle is hidden.

Note that the toggle just shows or hides the HTML video element; it does
not notify the remote participant to mute her video or to fully stop
sending it. It is purely a local change that does not affect the remote
clients. In the future this could be extended to involve the remote
clients too, but for now just hiding the HTML video element notably
reduces the CPU load in most systems (although unfortunately in some
cases it does not).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>