[BUGFIX] Fix the BC introduced by renaming the form finishers, also remove the default mail finishers

MailToSenderFinisher -> MailToUserFinisher
MailToReceiverFinisher -> MailToAdminFinisher
userName -> mailFromName

always ignore mail queue

- Added JS confirmation before sending the email
- Added a hidden field for better distinguishing the preview emails (there was a fluid bug)
- Added an extra groupBy to avoid sending multiple emails to the same recipient

Fixed:
- preview markers in the newsletter
- keep values in the newsletter form after sending
- fixed pagination performance in MailQueue

Sending to subgroup members
Markup changes

[TASK] Allow comma separated template marker form sg_forms to display meaningful labels in the all variable. Other bugfixes