Unable to edit or add new policies after project deletion
**Summary**:
Policies cannot be edited or added once the security policy project is deleted.
Steps to reproduce the issue:
1. Create a group and a project
2. Add new policy: I have tested with the merge request approval policy
3. Add a MR to confirm that the policy is working and that the approval is needed
4. As a group owner go to the Security project policy settings and delete the project
5. A message will be returned that the project is read only
6. While trying to edit or add new policy the following message is returned: You are not allowed to push into this branch
7.

8. Policies are still in place once the project was removed

Why this might be a bug:
- It could lock a group in the case of misconfigured policies and deletion of the policies project.
## Proposal
> prevent from deleting the project when it is linked as SPP to other groups/projects -\> we could list projects/groups and ask user to unlink them first before proceeding with deletion
epic