Refactor boolean validation specs

In many spec files, we have validation specs that look like this:

    it { is_expected.not_to allow_value(nil).for(:delete_unconfirmed_users) }
    it { is_expected.to allow_value(true).for(:delete_unconfirmed_users) }
    it { is_expected.to allow_value(false).for(:delete_unconfirmed_users) }

These 3 lines can be replaced with a single line:

it { is_expected.to allow_value(true, false).for(:delete_unconfirmed_users) } 

This is a refactor / cleanup that could be tackled by a newer contributor to GitLab. But there are many instances of this (I saw several in just one file: spec/models/application_setting_spec.rb) so it might be smart to fix this over the course of several MRs.