fix new mailer language strings (#837 related)
backend edit attendees send proper mails (relates #839)

In case admin changes an attendee unattend mails for old user and attend mails for new user will be sent.
(this requires sendemail field, so it's always shown)
backend: prevent doublicate attendee registration (relates #839)
Merge remote-tracking branch 'remotes/origin/development' into dev21