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 *important* comments, questions, and documentation can be found on a shared document here:
https://docs.google.com/document/d/1sOnan8QTOrAgzezXfOnf1tpl67y7MCUm4wnLrXGVzLw/edit?usp=sharing

@pmitros