assets/readme-rendering: Use dompurify as XSS filter
XSS filtering has recently been added to swh-web
(!89 (closed)) for the rendering
of README files in markdown format.
But as @kalpitk noticed it, the rendering of images located in an origin source tree is now broken.
So instead of using showdown-xss-filter
package, prefer to use the dompurify
one which seems to have a good default white list for XSS filtering.
Related #1642 (closed)
Migrated from D1412 (view on Phabricator)