fix XSS vulnerability in readme rendering for txt, md

Summary:
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

Reviewers: Reviewers

Differential Revision: https://forge.softwareheritage.org/D1322