Rename "Episode" class to "Chapter" or "Page"

[zkamvar]

During user testing for the new interface, users expressed confusion about the term "Episode", referring to individual chapters/units/files of a lesson. The terminology of "episode" is intended to reflect that each one should be no more than 15 to 45 minutes long (about the length of an episode in a television series), but that aspect is very nuanced. A better term is "chapter".

We should reconsider naming the class that contains each chapter/unit/file/page an "Episode" and call it either "Chapter" or "Page" to make it more semantically correct.