Added: Disable custom DNS records feature in case of external DNS server usage (Gui)

    [feedquote='i-MSCP Github (stable) (nuxwin)','https://github.com/i-MSCP/imscp/commit/009c7e963bfd3f64964332e3b5d715109800346f']

    Code
    1. Added: Disable custom DNS records feature in case of external DNS server usage (Gui)
    2. Added: External server option for DNS server (Installer)
    3. Fixed: Local resolver must be disabled in case of external DNS server usage (Installer)
    4. Fixed: When switching to external DNS server, uninstallation of current DNS server must be triggered (Installer)

    [/feedquote]