BS4 "btn-default" class was replaced with "btn-secondary", which makes more sense.
1. Splitted data/mass_mailing_data.xml to security/mass_mailing_security.xml for res_groups data. 2. Splitted views/mass_mailing_views to - views/link_tracker_views.xml - views/mass_mailing_stats_views.xml - views/mass_mailing_template.xml