Detect keyword collision in the same top level chapter

Currently, the converter assumed that a keyword could be used twice only in
different top level chapters, which is not true.
Now, links are generated with the more specific chapter level, which also
allows to suggest them in the search field.

This has currently some drawbacks :
- Links are more sensible to the addition of chapters in the future, which
  can break external links (A new heuristic to reduce the chapter level should
  be added).
- Search suggestions now provide the specific chapter title, which can
  sometimes be mysterious (This will be fixed by replacing the Bootstrap
  Typeahead component with Twitter Typeahead).