Add vulnerability management policy to selector

What does this MR do and why?

Related #465728 (closed)

Epic &5708

This adds the Vulnerability management policy type to the policy selector. It's guarded behind a newly introduced feature flag vulnerability_management_policy_type on the project level and vulnerability_management_policy_type_group on the group level.

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Before	After

How to set up and validate locally

1. Enable feature flags, go to http://gdk.test:3000/rails/features and enable vulnerability_management_policy_type and vulnerability_management_policy_type_group
2. In any project or group, go to Secure > Policies
3. Select New policy
4. Validate that there is a new tile for vulnerability management policy
5. Validate that if select Select policy, the query parameter type=vulnerability_management_policy is added.