Add table of contents.
  Add lesson header partial
  Add Lesson#lesson_number.
Chore: Refactor Lesson.published.
Update table of contents: link to new window.
  Set ids to 1 to 8.
Chore: Replace lesson number with id.
Chore: use basename as lesson link.
Update index: no need to pass lesson local to templates.
Add title to 'Open in new window' links.
Rename lesson basenames from 1 to 10.