WIP: Some changes in "Set up Postfix for Reply by email" page

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

I was setting up Postfix for reply by email and have run into several problems, which I succeeded in overcoming (see the commit message). I would like to share my solutions with other users.

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• If working on CE, submit an MR to EE with the changes as well.