Add missing kit rspecs, DRY up kit base items and report service

[jimmyli97]

Split off from PR #4585 Various refactors and documentation changes

Description

• Rename item.kits scope to item.housing_a_kit
• DRYs up kit base_item seeding
• Removes duplicated code and rspec from ChildrenReportService
• Add rspecs to:
  • prevent kit base_item deletion
  • test item.is_in_kit?, item.loose, item.housing_a_kit
• Add documentation

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

How Has This Been Tested?

passes test suite