[reader] Add Ajax-based navigation

[akprasad]

This modifies the reader to fetch section data with Ajax instead of a full-page request.

Pros:

• UI state is retained across sections. For example, an open sidebar persists across sections.

Cons:

• There's sometimes a flicker while JS content is rendering. I tried to minimize this by writing JSON data before the non-JS text data.
• Scroll position isn't retained properly. We track scroll position in the history state, but although setting it in the console works, setting it in the app doesn't. My guess is that the app is setting the scroll position before the main content has rendered.

Test plan: unit tests and clicked around on dev.