[lecture] improve css + navigation factorization + simplification in vie...

#### Lecture: fixes

:heavy_check_mark: Do the integration
:heavy_check_mark: Factorization for the navigation
:heavy_check_mark: Simplification of django view

:x: Download link doesn't work for now

#### Result

![capture d ecran 2014-10-02 a 14 17 21](https://cloud.githubusercontent.com/assets/2462303/4490618/68066db6-4a2e-11e4-8d54-5a4c13ccdbc4.png)