When saving a template in version 8.0, html would be saved as it should
be displayed once on the site. In particular, if some text should be
escaped once send to the browser, it will be saved as such.

But when rendering, a text node content is unescaped two times:

* for translation which seems wrong since we already use .text of a node
  which already escaped it, doing it one more time is bad,

* when rendering the template, since the html template is stored in xml,

This commit remove superfluous unescaping for translation, and add an
escaping when saving the changed template content.

closes #7967
opw-646889