Merge branch 'shell_version_install_doc' into '6-8-stable'

Update Shell version in the Install doc
Merge branch '6-8-stable' of https://dev.gitlab.org/gitlab/gitlabhq into 6-8-stable
Bump the required gitlab-shell version to 1.9.3
Bump version to 6.8.1
6.8
Demo disclaimer.
Add demo user.
Update promo.
Move alpha warning to login. Use with broadcast.
Motivate learn contributing.
Labels.
enhancement to feature-request
Design goal divergences
Make tree new file link more visible.
Point to CHANGELOG_FORK and accepting-pr.
gitlab wontfix and timeout labels.
howto label.
dep label.
gitlab-accepting-pr label
pressbooks.