[frontend] new generic web-hook
    
    This webhook is theoretically useable for the existing --srpm
    build methods, but we need to get the webhook payload parsers (for
    Github and Gitlab) concentrated on one place, and get the combo
    parser applied here, too (TODO task).
    
    The webhook payload is for now only stored in temporary data
    directory, so the end-user (via the future srpm build method) will
    have the payload in hand and can parse it manually.