Adding Precedense:bulk header to the Mailgun mailer
Adding Precedense:bulk header to the Mailgun mailer

Czech centre against SPAM marked this as one of the criteria to prevent
emails be marked as SPAM in Czech republic.

remp/remp#290

remp2020/remp,remp2020/remp,remp2020/remp,remp2020/remp,remp2020/remp