Bug 1019098 - Remove progressbar on FTU tutorial

* progressbar is hidden when there are <= 3 steps
* CSS adjustments to take up this space and allow for longer descriptions that accompany the videos
* vertically center the text in its fixed-height container
don't we prefer to use `display: none`? why only hide it and leave the space empty? even if we put elements over it, `collapse` doesn't make much sense