Release version 2.7.0

Features:
* Users can now delete their own account through the settings panel.
* Admins can now delete any account or session through the settings panel.

Improvements:
* CSV Export: A settings panel has been added. Semicolon and tab can be used
  as separator. An Excel-specific header can be added to improve compatibilty.
* The ability to set custom icons for authentication services via backend
  configuration has been restored.
* Export/import functionality is no longer disabled for iOS.

Bug fixes:
* The flip animation for flashcards has been disabled to fix rendering issues in
  some browsers.
* CSV export now works for flashcards.
* Cloning of sessions from the public pool has been fixed.

Known issues:
* CSV import is not supported for flashcards.
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