mail/rainloop: Add patch to fix cross-site-scripting (XSS) vulnerability

PR: 263749
Reported by: Lapo Luchini
Obtained from: https://blog.sonarsource.com/rainloop-emails-at-risk-due-to-code-flaw
MFH: 2022Q2
Security: a8118db0-cac2-11ec-9288-0800270512f4

(cherry picked from commit f9f524f160cb67555ffab240926b693d090ebd20)