[FIX] calendar/mail: correctly use registries

These two addons were doing evil things with the way registries works.
1. they create a new registry to add something in it,
2. they replace the existing reference with another
3. they don't use the "add" method.

Seriously, original coder, what were you thinking?

addons/calendar/static/src/js/base_calendar.js

@@ -266,9 +266,7 @@ function reload_favorite_list(result) {
             }
         }
     });
-    instance.web.form.widgets = instance.web.form.widgets.extend({
-        'many2manyattendee' : 'instance.web.form.Many2ManyAttendee',
-    });
+    instance.web.form.widgets.add('many2manyattendee', 'instance.web.form.Many2ManyAttendee');
 
     instance.calendar.event = function (db, action, id, view, attendee_data) {
         instance.session.session_bind(instance.session.origin).done(function () {

addons/mail/static/src/js/many2many_tags_email.js

@@ -81,8 +81,6 @@ instance.web.form.FieldMany2ManyTagsEmail = instance.web.form.FieldMany2ManyTags
 /**
  * Registry of form fields
  */
-instance.web.form.widgets = instance.web.form.widgets.extend({
-    'many2many_tags_email' : 'instance.web.form.FieldMany2ManyTagsEmail',
-});
+instance.web.form.widgets.add('many2many_tags_email', 'instance.web.form.FieldMany2ManyTagsEmail');
 
 };