Add a danger that checks if PreventIndexCreation cop is disabled

Overview

We have the Migration/PreventIndexCreation cop that prevents creating indexes for certain tables.

Sometimes, developers skip this cop without providing a concrete explanation about why that index is being added.

We have an established process that requires an approval to bypass the cop. https://docs.gitlab.com/development/database/large_tables_limitations/#requesting-an-exception

Deliverables

• Add a new Database Danger check that verifies that Migration/PreventIndexCreation was skipped, and requires the link to the exception issue.