[ADD] website_slides: new website module to handle presentations, documents,

videos, infographics. Some kind of slideshare clone, to summarize.

Featuring :
- channel management: classify slides (presentations, videos, documents, ...)
and tune their access rights for visibility, uploading and downloading
- full branding new shiny website module !
- automatic fetch of youtube and google docs information based on URL
- statistics: views, likes, interfaced with messaging, ...
- embed mechanism to embed slides in other websites

Courtesy of RD India, notably Mantavya Gajjar, Ravi Gadhia,  Vijay Patel,
Pariket Trivedi
Courtesy of Use Merge Team, notably Jerome Maes and Richard Mathot
Let us not forget our usability testing teams that tested it quite a lot of time.

Special thanks to Martin Trigaux that did nothing.