variable-width: Only set fixed height by default

Set player width to 100% by default, but player height to the default proportions for a 760px width. The player height doesn't automatically adjust to changing widths, and instead adds black lines to preserve the video proportions. With these defaults, we now allow for browser resizing, which can change the available width - the video will now reduce in size, and add black lines on top/bottom to compensate for the reduced width.

Also change variable names to video width/height to default values on existing courses.

@dragonfi @Tivoli Review/comments welcome