Update Title + Chapter model, refactor their APIs:

- Add Title model
- Chapter now has more attributes (title it belongs to, name, number)
- TitleHandler now populates its chapter list, though the page list for
  each chapter is not fetched yet.
- ChapterHandler looks up existing Chapter record and fetch page list to
  complete its info. Now this API returns the full info combined by the
  2 steps: name, number, next/previous chapter URLs