Allow more mail clients to group emails by thread

* send a 'In-Reply-To' header along the 'References' header
* subject of answers to an existing thread begins with 'Re: '
Add sanitize option to gitlab check rake task.
Remove duplicated calls.
Add sanitize option to documentation.
Merge branch 'sanitize_rake_check_output' into 'master'

Sanitize rake check output

Fixes #1206

See merge request !952
Merge branch 'improve-email-threading' into 'master'

Improve email threading

This is a follow-up to !112 : it improves email threading, without changing the receiver of the emails.

* Subject of answers to an existing thread begins with `Re: ` (required by Mail.app)
* Send a 'In-Reply-To' header along the 'References' header
Merge branch 'master' of gitlab.com:gitlab-org/gitlab-ce