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

tpl/transform/transform_test.go

@@ -244,6 +244,6 @@ func TestPlainify(t *testing.T) {
 		}
 
 		b.Assert(err, qt.IsNil)
-		b.Assert(result, qt.Equals, test.expect)
+		b.Assert(result, qt.Equals, template.HTML(test.expect.(string)))
 	}
 }