Adds a third CS exercise challenging the user to place a colored circle
on a background of a different color.  While the exercise checks for the
required function calls, their order is not yet verified.  A potential
point of confusion is that, when asked to draw a blue circle, the student
might think that any shade of blue is acceptable, but only pure blue is
currently accepted.  Consider clarifying the instructions to the student
or accepting a range for each color.

This commit also adds a file utils/colors.js providing a list COLORS of
objects with name, red, green, and blue properties.  This may be useful
for other exercises.

This commit incorporates all of the changes in patch 34693.patch.