[FIX] mass_mailing: set theme without changing template content

When opening a mail template,
and choosing a theme,
then directly saving, without changing anything within the template,
the chosen template wasn't saved in the template.

Therefore, when re-editing the same template, the theme selection
re-appeared, while you had already chosen in the previous steps.

opw-665537

addons/mass_mailing/static/src/js/mass_mailing_editor.js

@@ -193,6 +193,7 @@ web_editor.Class.include({
     set_snippet_theme: function (theme) {
         $("#o_left_bar .o_panel_body > div").addClass("hidden");
         $("#o_left_bar .o_panel_body > div."+theme).removeClass("hidden");
+        $("#editable_area").trigger("content_changed");
     },
     get_snippet_template: function (mailing_model) {
         var self = this;