Sketch_type

Added a link from Category page to specific Sketch_type which displays
the list of the videos referenced for this sketch_type
Added a link from Sketch_type page to specific Sketch which doesn't
display anything for now

Modified routes :
- serie/(:num) : leads to the page of that specific sketch_type
(identified by it's "id" in second segment)
- category/(:num) : leads to the page of that specific category
(identified by it's "id" in second segment)