lib: sanitize HTML for all types of README rendering, not only markdown The repository summary page will display a rendered version of the repository 'readme' based on its file extension. In commit 5746cc3b3fa5, the rendered output was already sanitized when the input was markdown. However, also readmes written in other formats, like ReStructuredText (RST) or plain text could have content that we want sanitized. Therefore, move the sanitizing one level up so it covers all renderers, for now and the future. This fixes an XSS issue when a repository readme contains javascript code, which would be executed when the repository summary page is visited by a user. Reported by Bob Hogg <wombat@rwhogg.site> (thanks!).