Change location and refactor issue closing pattern documentation

Split into user and administrator docs:

• administration/issue_closing_pattern.md
• user/project/issues/automatic_issue_closing.md

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6466

Status changed to merged

Mentioned in commit ce3cfac4

Picked into 8-12-stable-ee, will go into 8.12.0.

Removed ~149424 label

Mentioned in commit 373ad85a