refactor group model validations, add db constraints and unique index on identifier
remove validates_associated from chore
add valid_attribute to gemfile, bundle
add valid_attribute to spec_helper.rb
refactor group spec to use valid_attribute
refactor chore specs to use valid attribute
refactor debt specs to use valid_attribute
add unit test for debt search method
refactor debt specs
refactor user and debt unit specs to use valid_attribute
add null: false to user name and group_id