Improve expand/collapse feature

[drikusroor]

This PR aims to:

• Simplify the expand/collapse feature by adding one button in the top bar to toggle all sections at once
• Persist the toggle state in a url parameter named summary
• Add aria labels to the button that reflect the current state of the toggle
• Use a data attribute instead of a class to select the toggle button
• Minor updates to the CSP comments

Resolves #73

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[jeroenheijmans]

Initial glance at the Deploy Preview: this is exactly what we were looking for IMO! Nice work 👍🏻

I have some small suggestions in mind still (but nothing huge) feature wise, will sit down a bit later to write them down. I see you also asked @LucaScorpion for a review so I'll rely for him to provide the code review (in addition to any feature changes he might suggest).

[LucaScorpion]

Overall some great improvements! Just some small questions/considerations in terms of code. I do have one slightly bigger concern about the expand/collapse button content, but we could also consider this good enough for now and address that later.