The OpenLayers JS/CSS download URL is dead. Update it to make the map
work again:

- Update from OpenLayers 5.2.0 to 5.3.0
- Switch from the obsolete rawgit.com URL to jsdelivr.net (rawgit.com
  was only redirecting to jsdelivr.net for the last few years anyways)
- Set a fixed commit in the URL, so the URL doesn't become outdated again