[FIX] web_editor: make html widget translatable

The translate feature of widgets html and html_frame had been
broken by the new views refactoring. This commit restores it for
the html widget, and removes it completely for html_frame as it
wasn't working correctly before the new views anyway (moreover,
it had already been disabled for mass_mailing: de403dc1).

This commit also ensures that the changes are correctly saved for
the html_frame widget. Note that this widget still needs to be
converted to our new coding guidelines.