tpl/transform: Make Plainify and ToMath return template.HTML

None of these are useful as plain strings in the templates, which forces the users to do `transform.Plainify "foo" | safeHTML`.

If people have trust issues with the output of these functions, they need to just stop using them.

Closes #8732

content/en/functions/transform/Plainify.md

@@ -6,7 +6,7 @@ keywords: []
 action:
   aliases: [plainify]
   related: []
-  returnType: string
+  returnType: template.HTML
   signatures: [transform.Plainify INPUT]
 aliases: [/functions/plainify]
 ---