Display icon for issues created by banned users in issue header

frontend polish for #327355 (closed)

Similarly to how confidential issues are clearly indicated by the eye-slash icon on the issue list, issue board, and the issue itself, hidden issues should be indicated by the spam icon on the issue header of the detail view.

Hidden issues are currently correctly indicated with the spam icon on the issue list, but not on the header of the detail view.

Implementation plan

1. Refactor app/views/projects/issues/_issue.html.haml#L15 into hidden_issue_icon
2. Use hidden_issue_icon in issuable_meta