Str2html was abused a lot. So use a proper name for it: SanitizeHTML
And add some tests to show its behavior.
(cherry picked from commit fb42972c057364a1dc99dfb528554e7a94415be7)
Conflicts:
docs/content/administration/mail-templates.en-us.md
docs/content/administration/mail-templates.zh-cn.md
prefer their version always