feat(bd-2g50): fill data gaps in issue detail view

Add references_full, user_notes_count, merge_requests_count computed
fields to show issue. Add closed_at and confidential columns via
migration 023.

Closes: bd-2g50

migrations/023_issue_detail_fields.sql

@@ -0,0 +1,5 @@
+ALTER TABLE issues ADD COLUMN closed_at TEXT;
+ALTER TABLE issues ADD COLUMN confidential INTEGER NOT NULL DEFAULT 0;
+
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (23, strftime('%s', 'now') * 1000, 'Add closed_at and confidential to issues');