In some cases ActiveSession.cleanup was not called after authentication,
so for some user ActiveSession lookup keys grew without ever cleaning
up. This Rake task manually iterates over the lookup keys and removes
ones without existing ActiveSession.

- Move steps to new doc
- Move all concepts to different docs

Removing extra blank lines in docs that serve
no purpose, as well as some --- lines that also
are not needed.

Hard tabs do not follow general markdown guidelines
are were removed from the few docs that used them

Updating redirected links in the development, install, policy
raketasks and university dirs. Should now link to final
URL to avoid redirect hops

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

Change assorted absolute links to ce and ee docs
to relative links

Add rake task to clean orphan artifact files

See merge request gitlab-org/gitlab-ce!29681