Leave serve_xss_vulnerable_mimetypes as default

This image seems to be quite useful for some people. As suggested by @jmchilton I will leave that setting as it is recommend by Galaxy.

Please add the following line to your Dockerfile if you would like to render SVG images.

``RUN sed -i 's|#serve_xss_vulnerable_mimetypes.*|serve_xss_vulnerable_mimetypes = True|g' ~/galaxy-central/universe_wsgi.ini``