Fix messed up custom fields in settings
    
    The new tabbed layout of the settings page changed the way
    we add rows to some of the settings that use rows, like priorities
    and the custom fields. This new way used a hidden blank-field that
    allowed us to duplicate. However, this field was sticking around when
    submitting the form, and was causing the custom fields to not be
    submitted propertly to the post call. This PR uses javascript to remove
    the hidden field from the form before it is submitted.
    
    Relates to #3366
    
    Signed-off-by: Ryan Lerch <rlerch@redhat.com>