fix XSS vulnerability in readme rendering for txt, md
While rendering readme, scripts were also being executed. Example - https://archive.softwareheritage.org/browse/origin/https://github.com/kalpitk/test/directory/ and https://archive.softwareheritage.org/browse/revision/8428612a5f8d115deff9463fdff6da62d2fc6091/?origin=https://github.com/kalpitk/test
Migrated from D1322 (view on Phabricator)