Add statistics route, permissions, and sidebar entry

- /statistics route and PagePaths.statistics
- Permission sets: viewer and admin can access /statistics
- Sidebar link with can_access_page check
- Plug and sidebar tests updated

docs/page-permission-route-coverage.md

@@ -26,6 +26,7 @@ This document lists all protected routes, which permission set may access them,
 | `/groups/new` | ✗ | ✗ | ✗ | ✓ |
 | `/groups/:slug` | ✗ | ✓ | ✓ | ✓ |
 | `/groups/:slug/edit` | ✗ | ✗ | ✗ | ✓ |
+| `/statistics` | ✗ | ✓ | ✓ | ✓ |
 | `/admin/roles` | ✗ | ✗ | ✗ | ✓ |
 | `/admin/roles/new` | ✗ | ✗ | ✗ | ✓ |
 | `/admin/roles/:id` | ✗ | ✗ | ✗ | ✓ |