Refactor App and HeaderApp to merge common functionality

[gsidebo]

This is a holdover from mitxpro, which suffers from the same debt.

App.js and HeaderApp.js have a large amount of duplicated code, and almost every time we add functionality to one, we have to do the exact same for the other. Aside from the extra code maintenance, we run the risk of these two files diverging and experience unexpected behavior and bugs between the CMS- and React-based portions of the app. We should refactor them to abstract away the common logic.

[gsidebo]

Related issue in mitxpro: mitodl/mitxpro#500