Create feature flag for DAST Pre-scan verification epic

Create feature flag :dast_pre_scan_verification in ee/config/feature_flags/development/dast_ui_redesign.yml

Add feature flag to ee/app/controllers/projects/on_demand_scans_controller.rb

You can use this merged MR as a reference

• Team: Secure:Dynamic Analysis
• Most appropriate slack channel to reach out to: #g_secure-dynamic-analysis
• Best individual to reach out to: Artur Fedorov, Dheeraj Joshi
• PM: Seth Berger