Revise first two blockchain lessons

[JoshOrndorff]

This PR makes a few small changes to the first two lessons. Despite the changes not being very substantial, the diff is large.

• Move images into clearer hierarchy (large diff, little substance)
• Bring content back from contracts module (large diff, little substance)
• Fix incorrect state root in diagram
• Add link to shared storytelling talk
• Revise block header slide to better compare fields and clarify we are talking about the old eth pow, not the new pos one.
• Add audio amplifier state machine example. (plus I added the "answer" to my question into the speaker notes like we discussed the other day.)