Merge: Opportunity and Tnitter logos

This PR intro 2 logos for Opportunity (in English and French) and a new wide icon for Tnitter. These news icons are already in use on http://xymus.net/opportunity.

Note that even if the large Opportunity logo on the landing page is vectorial, it should not be the file included in the `art/` directory for a production server. That file has both logos, more assets and uses fonts, all of which should be stripped for a production server. It is still good enough for local debugging.

Pull-Request: #1959
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>