Improved wording of German translations in extract attachments view.

Also renamed some i18n message IDs and defaults to better reflect their supposed message.
Merge pull request #436 from 4teamwork/lg-improve-extract-attachments-wording

Extract attachments: Improve consistency in wording
Added new mail event `AttachmentsDeleted`.
Synced translations for opengever.mail.
Journalize AttachmentsDeleted event.