Re-emplementing encryption using Sodium

Changes:
- Encryption using Sodium
- Key is generated using random_bytes() (cryptographically secure)
- Encryption is done using a second file, which is renamed after encryption is
  complete (by using the same file, we would encrypt already encrypted data
  again)
- A file encrypted using Sodium is marked with "C2" in the link file, so we can
  distinguish them from files encrypted using mcrypt

ToDo:
- Error checking
- Show a warning in the admin interface if Sodium is not available