Merge pull request #431 from 4teamwork/lg-extend-mail-actions

Extend mail actions with common actions for regular documents
Merge pull request #430 from 4teamwork/lg-disable-file-or-paper-validator-for-mails

Make FileOrPaperValidator consider mails properly.
Modified mail metadata test so it reproduces encoding issue in resolving mail author:

- Use different names for OGDS user, From: header and To: header.
- Make sure both From: and To: contain non-ASCII characters
- Assert on actual values instead of using the same function the code to
  be tested uses.
Fix UnicodeError in resolving mail author (if author not an OGDS user).