Prevent custom role from being deleted if it is used in security policies

Why are we doing this work

• a custom role can be modified or deleted and a security policy could be affected by that

Relevant links

• &13550 (comment 2235834119)
• &13550 (comment 2263942526)

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• frontend / backend disable the Delete role button if they are used in a policy
• frontend / backend show a tooltip with the existing policies and links to them

Similar to the override policy warning icon recently created

Verification steps

1. Upload a GitLab Ultimate license
2. Enable the feature security_policy_custom_roles feature flag
3. Navigate to the admin level or top-level group => Settings => Roles and permissions
4. Create a custom role
5. Navigate to a group/project under that => Secure => Policies => New project => Merge request approval policy => Create a policy with the custom role as an approver
6. Add a member as having that role
7. Create a merge request and verify the approver appears