Disallow Single Table Inheritance in GitLab application

Following https://docs.gitlab.com/ee/development/database/single_table_inheritance.html, write a cop to disallow Single Table Inheritance

Why ?

Every addition of use of a Single Table Inheritance likely is it is adding to Instance / Group / Project types, and hence slows down classification of gitlab_main tables because of mixed usages. These mixed usages also block cells because we will need to fix them as well

e.g:

• labels, see Admin::AbuseReportLabel