https://trello.com/c/bFMJWmLu/2251-sgmail-sgforms-anpassungen
If no custom template key was entered by the editor, a mix of the form & finisher identifier was used, to uniquely identify the mail template. This commit makes sure this value is also saved in the form configuration, so that the editor immediately sees the generated template name in the backend form editor.

always ignore mail queue

- Fixed PHPDoc
- Fixed the runtime template cache
- Fixed coding styles
- Changed preg_match to strpos for better performance

- Added static runtime cache

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

To save performance, we exclude deleted records from the update process,
because it is unnecessary to process such records.