Reimplementing the slide editor lay-out with flexbox (4)

In the most recent versions of Firefox, switching from a slide with a question to a slide without a question in the Quiz Editor breaks the lay-out. The change PDF page button and delete slide button remain too large causing the view to not render correctly.

The whole slide editor part of the Quiz Editor is problematic anyway. It uses ton of resize and state change events to stay synchronized. Almost all of it could have been prevented if we would have used flexbox. However, this was not used in Quizzard until the very end. Now might be the best time to fix the slide editor issues once and for all.

To do:

• Make an equivalent lay-out for the slide editor using flexbox (2)
• Remove all older resize and state change events (1)
• Remove all older CSS hacks (1)