bf3de18 frontend: process external repos for custom build

Authored and Committed by mfocko a year ago
    frontend: process external repos for custom build
    
    - Pass external repos acquired from the API endpoint.
    - Add text area for external repos when triggering custom build from the
      web view.
    
    Related to #530
    
    Signed-off-by: Matej Focko <mfocko@redhat.com>