Stimuli are rendered with the correct opacity

In each scene the opacity is taken from the current size of
the relevant moon.
Links that are not opaque are inactive

They display/behave like normal text, save for color.
Based link clickability off link opacity

I was doing this silly run-around to get the correct moon for the sense.
Text that is no longer visible has no cursor effect
Checked off a few irrelevant todos
Made a seeds.rb file that parses yaml

Also created seeds.yaml as an example