hgweb: change highlighted line color to be different from 'inserted' color
This changes line highlight color from a fain yellow (#ffff99) to
a faint blue (#bfdfff), because yellow color is used in comparison view for
inserted lines.
This new color is okay for people with different forms of color blindness
(tested with a simulator):
a) this color looks quite different from other used backgrounds
b) text doesn't lose distinction on this color