Ease migration of existing books

Functionality to import table of contents to json format from an extisting contents page of a book.

Bug: 53286
Change-Id: I392a3c4bebbf8325d33db3b5d62d5d415d47da4f
Import chapter list from existing book in the same order as on the contents page

Create an api module for returning internal links in the same order as they appear on the page.

Bug: 66367
Change-Id: Icac6a7a40925e6aef70d4b963b6ba476504d3141
Merge changes Icac6a7a4,I392a3c4b

* changes:
  Import chapter list from existing book in the same order as on the contents page
  Ease migration of existing books