Change Content-Security-Policy to allow base64 data images

To be able to use base64 data images (for example <img src="data:image/png;base64...") the directive img-src 'self' data:; must be added to https://github.com/nuxeo/nuxeo/blob/d4e1461d130b814c20fc39d4804f4318ee1471d5/nuxeo-services/nuxeo-platform-web-common/src/main/resources/OSGI-INF/web-request-controller-contrib.xml#L53