Changed templates of mail and adressbook to use one base template
Removed temporary files which should not be tracked.
Adjusted tests to renaming of views
mailbox selection in message view added
added calendar app with corresponding models
added admin for calendar app
added choice for series or single appointments
extended mail model
modified helper and view for new message model
mails have unique identifiers and additional information
added calendar crud views
merged development and devel-backend
Removed files which should not be tracked
Removed selenium tests as they fail until frontend changes are completed