'Click forward button at specified `source_index`.'

def click_forward_button(self, source_index):

    css = '.move-modal    .xblock-items-container    .xblock-item'
    self.q(css='.button-forward').nth(source_index).click()
    self.wait_for((lambda : (len(self.q(css=css).results) > 0)), description='children    are    visible')
