Fix slides bugs

[JaroCamphuijsen]

• fragments are flaky, they sometimes work and sometimes don't often when starting a presentation fresh it does not work and already loads the next slide (setting its class to present), while the fragment is set to visible. Because the fragment's parent (the previous slide) is now set to have class "past" and so is hidden, with the fragment not appearing at all.
• Sometimes slides' content is positioned weirdly offset towards the bottom upon a first view. Often it is fixed by going back and forth.

[DaniBodor]

Also, sometimes images do not center by default. I tried to add this slide (in 2 ways) but the image keeps showing left aligned.
Oddly, on the first view (on reloading the page), while the entire content of the slides appears too low (lots of empty space on the top) and bullet points are poorly formatted, the images are actually centered as expected. Then, after going back and forth in the slides, while other issues are 'fixed', the image alignment suddenly (sometimes) changes to left alignment

<!-- .slide: data-state="standard" -->

## Software is also data

<div style="text-align:center">
  <img src="media/data_definition.png" width="70%" justify="center">
  <small>Definition by <a href="https://www.merriam-webster.com/dictionary/data/">Merriam Webster English Dictionary</a></small>
</div>

==

<!-- .slide: data-state="standard" -->

## Software is also data

<img src="media/data_definition.png" width="70%" justify="center">
<small>Definition by <a href="https://www.merriam-webster.com/dictionary/data/">Merriam Webster English Dictionary</a></small>

==

First view:

Final view: