The current Skosmos UI uses Bootstrap CSS version 3.

We should upgrade to Bootstrap 4, which among other things uses flexbox for layout. This should make it easy to fix alignment issues such as those described in issue #1006.

Upgrading Bootstrap is a large undertaking as all the HTML templates and CSS files are affected.

Bootstrap 4 drops support from some older browsers. We should verify that this doesn't cause problems for users of important Skosmos installations such as Finto.