WIP: Add Exchange Reply by Mail documentation

What does this MR do?

This is a continuation/replacement of gitlab-org/gitlab-ce!1624. It provides documentation on setting up Microsoft's Exchange server to work with the reply by email feature.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together