- 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

This commit adds proper error messages when mail sending via the
mail queue has failed. It also refactors the MailTemplateService that it
does not use duplicate code to send mails.