WIP: Changing the voice, grammar, and content structure

Large editorial changes to voice, grammar, and content structure of the course. 

Work in progress. Still editing last few sections. 

Additional comments, questions, and documentation can be found on a shared document here:
https://docs.google.com/document/d/1sOnan8QTOrAgzezXfOnf1tpl67y7MCUm4wnLrXGVzLw/edit?usp=sharing