[extract] try to extract a relevant part from the html + clean the view ...

##### extract

:heavy_check_mark: (Little) clean of the manage_book_synth view (remove useless code)
:heavy_check_mark: Improvement on the `detail` page using less variable in the loop
:heavy_check_mark: Add new properties to the `Synthese` model => `extrait`
:heavy_check_mark: Implement the logic to transform html into an extract


##### Result
The total content of the two syntheses corresponds respectively to the wikipedia for The Lean StartUp and the documentation of BeatifulSoup (which is huge).

![](http://g.recordit.co/9YYsqWsCPq.gif)