Disable "Unprotect" button when policy is in place

What does this MR do and why?

This MR resolves #421623 (closed)

Protected branches can not be unprotected if security policy is in place.

Security policy set up can prevent users from unprotecting branches

This MR also adds a popover

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Upload a GitLab Ultimate license
2. Enable the feature flags

echo "Feature.enable(:scan_result_policies_block_unprotecting_branches)" | rails c

3. Go to Project/Settings/Repository/Protected Branches /settings/repository#js-protected-branches-settings
4. Protect couple of branches, you should be able to unprotect it
5. Go to Secure -> Policies -> New policy -> Scan Result Policy
6. Go to yaml mode
7. Use this policy:

   type: scan_result_policy
   name: Container Scanning
   description: ''
   enabled: true
   actions:
   - type: require_approval
     approvals_required: 1
     user_approvers_ids:
     - 1
   rules:
   - type: scan_finding
     scanners:
     - container_scanning
     vulnerabilities_allowed: 0
     severity_levels: []
     vulnerability_states: []
     branch_type: protected
     branch_exceptions:
     - foobar (actual protected branches name you want to exclude)
   approval_settings:
     block_unprotecting_branches: true

8. Go back to settings, unprotect buttons should be disabled and popover for disabled buttons should be in place on hover

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #421623 (closed)