Security update for enigmail (moderate)

This update for enigmail to version 2.0.4 fixes multiple issues.

Security issues fixed:

• CVE-2017-17688: CFB gadget attacks allowed to exfiltrate plaintext out
  of encrypted emails. enigmail now fails on GnuPG integrity check
  warnings for old Algorithms (bsc#1093151)
• CVE-2017-17689: CBC gadget attacks allows to exfiltrate plaintext out of
  encrypted emails (bsc#1093152)

This update also includes new and updated functionality:

• The Encryption and Signing buttons now work for both OpenPGP and S/MIME.
  Enigmail will chose between S/MIME or OpenPGP depending on whether the
  keys for all recipients are available for the respective standard
• Support for the Autocrypt standard, which is now enabled by default
• Support for Pretty Easy Privacy (p≡p)
• Support for Web Key Directory (WKD)
• The message subject can now be encrypted and replaced with a dummy
  subject, following the Memory Hole standard forprotected Email Headers
• keys on keyring are automatically refreshed from keyservers at irregular
  intervals
• Subsequent updates of Enigmail no longer require a restart of Thunderbird
• Keys are internally addressed using the fingerprint instead of the key ID