Phase 2 implementation:
- Add SHOW_IN_SEARCH constant (value 8) to Attribute model
- Update Items controller to include searchable attributes when
search_custom filter is enabled
- Add language strings for the new visibility flag
- Update Attributes controller to include new flag in form
This allows attributes to be searchable even if they are not
displayed in the items table, addressing issue #2919.
Fixes#2919
c53f862161