Personalization of Codabench front page with logos of servers hosting organizations

To better publicize the hosting servers organization of an instance, their logos are added on the front page:

• White version of the logos will be inserted aligned right, in the top banner of Codabench (#1219):
• The url of each institution is linked to the logos (#1219): www.lisn.upsaclay.fr www.universite-paris-saclay.fr www.cnrs.fr
• Documentation of the feature in the wiki should allow any Codabench instance admin to personalize his front page with his own logos/urls, as a new production server setting step: https://github.com/codalab/codabench/wiki/How-to-deploy-Codabench-on-your-server