Purpose of this task is to allow marketing people to differentiate the
mailing name (internal reference) from the subject used in the mailing
emails.

Mailing name is actually the UTM source name as mass mailing inherits
from it. Being able to edit it independently from the subject allows
to better categorize / filter mailings without sending technical terms
to customers. Marketing users could also change and tweak mailing subject
without disorganizing the pipe and changing the URM source name.

Demo data are updated accordingly to have both subject and mailing
names.

This commit is linked to task ID 1917602 and PR #29514.