Add docs for confidential issues

doc/user/permissions.md

doc/user/project/issues/confidential_issues.md

doc/user/project/issues/img/confidential_issues_create.png

...ser/project/issues/img/confidential_issues_index_page.png

...ser/project/issues/img/confidential_issues_issue_page.png

...r/project/issues/img/confidential_issues_search_guest.png

.../project/issues/img/confidential_issues_search_master.png

...r/project/issues/img/confidential_issues_system_notes.png

doc/workflow/README.md