Cleanup, update and fix backend – refactor Chapters 1–8

- Refactor the backend, fix some botches (merge conflict in the stylesheet, for example…) and make config/README up-to-date
- Cleanup templates, cleanup/fix HTML tags misuses
- Refactor Chapters 1–8 (i.e. rewrite a few parts and use smart quotes) and wrap them to 100 lines (the rest is TODO)
- Fix various issues in some other pages and use HTML char entities
- Minor stylistic changes: make the text a bit darker for better contrast (and so better readability), reduce slightly the headers sizes to not harness the reader, reduce slightly the header text on small widths to fit the container, misc other changes
- Various refactorings

r? @davelab6