FE: Add `Prevent pushing...` policy editor setting

Why are we doing this work

• users want to override protected branches setting with a Prevent pushing and force pushing checkbox on the scan result policy editor

Relevant links

Non-functional requirements

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

Implementation plan

• add a new checkbox under protected branches that toggles the prevent_force_pushing: true yaml setting
• hide it behind the scan_result_policies_block_force_push feature flag

Verification steps