Switch from CSS to Table layout for HTML mails so they work in Outlook aka Word

Remove templates-vector and theme templates with variables instead
Switch to matrix.to URLs by default for links

res/templates/mail-Vector.css

+.header {
+    border-bottom: 4px solid #e4f7ed ! important;
+}
+
+.notif_link a, .footer a {
+    color: #76CFA6 ! important;
+}