Merge branch 'rfc-3676-mailsig' into 'master'

set proper signature marker in mails

Closes #43617

See merge request gitlab-org/gitlab-ce!17979

app/views/layouts/mailer.text.erb

 <%= yield -%>
 
----
+-- <%# signature marker %>
 You're receiving this email because of your account on <%= Gitlab.config.gitlab.host %>.

app/views/layouts/notify.text.erb

 <%= yield -%>
 
----
+-- <%# signature marker %>
 <%  if @target_url -%>
 <%    if @reply_by_email -%>
 <%=     "Reply to this email directly or view it on GitLab: #{@target_url}" -%>

changelogs/unreleased/43617-mailsig.yml

+---
+title: Use RFC 3676 mail signature delimiters
+merge_request: 17979
+author: Enrico Scholz
+type: changed