enableFormelementSelectionButton in FinisherSetupBE.yaml for fields: replyTo, bcc & cc for both finishers

also set 'enableFormelementSelectionButton' to true for some fields

[BUGFIX] Fix broken file reference attachments

See merge request !19

This commit fixes FileReference attachments of multiple consequtive mails
that are sent via the mail queue, for example via the forms finisher with
a user and an admin mail, where files need to be attached to both mails.

- 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

[FEATURE] Add Mail Layouts

See merge request !17

[BUGFIX] Fix global variable name in README

See merge request !16