mailman security update

3:2.1.15-26.1 - Related: 1545974 - Add import regular expression module 3:2.1.15-26 - Related: 1545974 - Bump release to make it higher than 7.5 3:2.1.15-24.2 - Resolves: 1545974 - Add sanitizer to mitigate XSS injection 3:2.1.15-24.1 - Resolves: 1545974 - Fix XSS vulnerability in web UI...