Fix links as discovered by the lint in the docs project

Correct the spacing of unordered markdown lists
in docs, to maintain standards of documentation,
in /user and others.

Remove unneeded trailing whitespace in lines in
/security /workflow /user docs

We fetch the users from Phabricator based on their Phabricator ID. If
a user with the same username exists and is a member of the project,
we set them as assignee or author.

When a user is applicable, we also cache it in Redis so we don't have
to perform the request again for the same phid.

This updates redirected links in integration, update, user
and workflow dirs. Should now link to final URLs 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

Linting rule not enabled for now
because tooling produces false
positives.