backup the encryption key after the encryption was disabled

Keep a backup of the encryption keys after the encryption was disabled and the user decrypted his files.

This just adds another layer to prevent data lose. After the user decrypted all his files he can check if all files are readable again. If something went wrong he can restore the encryption keys to try to decrypt the files again. If everything is OK he can clean-up his data folder by permanently delete the backup.

Steps to test:
- enable encryption
- logout/login
- upload some files
- filekeys and sharekeys should be in ```data/user/files_encryption```
- disable encryption
- go to the personal settings to decrypt your files
- now the keys should be moved to ```data/user/files_encryption/keyfiles.backup``` and ```data/user/files_encryption/share-keys.backup```
- you should see now two additional buttons to restore the encryption keys and to delete them permanently
- the delete button should delete the backup folders
- the restore button should move them back to  ```data/user/files_encryption/keyfiles``` and ```data/user/files_encryption/share-keys```