SCIM uses fallbacks when name.formatted not present

What

When SCIM users are being created and name -> formatted isn't present we fall back first to combining first/last names and then to displayName.

Why

Makes azure configuration less tricky and will be needed for Okta

See https://tools.ietf.org/html/rfc7643#section-4.1.1 for specification

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/13093 and https://gitlab.com/gitlab-org/gitlab-ee/issues/13016

Conformity

• Changelog entry
• Documentation created/updated. See: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31208/diffs