DAST Saved Scans - On-demand scans form - DastScan model

• frontend counterparts: #295241 (closed), #295242 (closed)

• Create DastScan model
• Create edit action
  • Should retrieve the scan by ID and pass its data to the frontend via data attributes in the HAML.
• Update ee/app/controllers/projects/on_demand_scans_controller.rb to retrieve an actual DastScan instance to be passed to the frontend (ping @pgascouvaillancourt to verify this).
• Update ee/spec/requests/projects/on_demand_scans_controller_spec.rb to use an actual DastScan in the #edit test case.