Added X-GitLab-... headers to emails from CI and Email On Push services

Fixes #2098

This adds the 'X-GitLab-Project', 'X-GitLab-Project-Id' and
'X-GitLab-Project-Path' headers to emails from CI and Email On Push in a way
that it is done currently for merge requests and issues emails.

Additionally, CI emails will have 'X-GitLab-Build-Status' header with either
'fail' or 'success'.

Emails from Email On Push will include 'X-Gitlab-Author' header containing the
username of user who did the push.

See merge request !2159