Provide option to allow replaying without chat messages

[olberger]

It seems that the chat may be displayed only if its input form element #exposechat is checked... to be tested

[trahay]

Another solution would be to add a blank mask over this part of the video with ffmpeg.

It should be quite easy according to : https://superuser.com/questions/1221633/ffmpeg-setting-a-rectangle-in-a-video-to-black