replace default presentation in new meeting

replace pptx and pdf which is shown at any new meeting, see https://docs.bigbluebutton.org/2.2/customize.html#change-the-default-presentation

it should have a start page with fairteaching logo and the features list and 5 empty pages for whiteboard (name them at the bottom: "ideas", "scrum board", "results", "later", "just for fun")