[Translators] Russian translation.

[Mikhail Kurinnoi]

Russian translation for v3.13.0
http://www.viewizard.com/clawsmail/v3.13.0_ru.po.gz

- Old ru.po file from git HEAD used as base.
- Translation updated to v3.13.0 with claws-mail.pot from v3.13.0
  release package.
- Mess with 'couldn't/can't', 'authorization/authentication',
  'security/privacy', etc fixed.
- Misspell fixed.
- All text revised (see diff with old ru.po file for more info).


Please note, I am not an professional translator, but software engineer.
If possible, someone more qualified should check it out.


Best regards,
Mikhail