Make favicon.ico available in web root

[imphil]

Since pre-historic times, it was assumed that the favicon is available at https://www.librecores.org/favicon.ico. We should make one version of the icon available there. This will also help our logs, which are currently full of messages like

[2020-07-29 10:10:22] request.INFO: Matched route "librecores_project_repo_user_org_view". {"route":"librecores_project_repo_user_org_view","route_parameters":{"_controller":"App\\Controller\\DefaultController::userOrgViewAction","userOrOrganization":"favicon.ico","_route":"librecores_project_repo_user_org_view"},"request_uri":"https://www.librecores.org/favicon.ico","method":"GET"} []
[2020-07-29 10:10:22] security.INFO: Populated the TokenStorage with an anonymous Token. [] []
[2020-07-29 10:10:22] request.ERROR: Uncaught PHP Exception Symfony\Component\HttpKernel\Exception\NotFoundHttpException: "User or organization not found." at /var/www/lc/site/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Controller/ControllerTrait.php line 356 {"exception":"[object] (Symfony\\Component\\HttpKernel\\Exception\\NotFoundHttpException(code: 0): User or organization not found. at /var/www/lc/site/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Controller/ControllerTrait.php:356)"} []