[FIX] mass_mail: Make it working save & continue button on the frontend ...

...to design a email (mass mailing) and attachments that were not sent in mass mailing (Mainetenance: 614563)
Alerady saw this code in master ... I think this is not the good way to fix the bug. The button should simply not be active in edit mode. It is not linked to the editor bar; this one should not be aware of things related to some custom page.