luci-mod-status: channel_analysis: use svg grad

Instead of using a second, semitransparent quadliteral of the
same station color to highlight the main 20 MHz channel of a
station with a bandwidht wider than 20 MHz, use a semitransparent,
black svg gradient with the same shape and size.
This has the benefit that it is harder to mistake for a second,
overlain station.

The gradient is black because this way it can be reused for
all stations, instead of having to create a separate one for
each station color.

Signed-off-by: Frank Löffler <frank.loeffler@uni-jena.de>