♿ [#1608] Setting up focus trap for accordion and mobile menu

[jiromaykin]

https://taiga.maykinmedia.nl/project/open-inwoner/task/1608

• make mobile menu the only Tabbable focus, hide everything else
• make accordion FAQ titles the only focus on Tab key when collapsed/hidden; then again make accordion-panel focusable when answer is open, so user can tab towards links inside the panels.

info: the mobile 'view' will become visible when users zoom in more than 200% - hence why this needs to be accessible with keyboard tabs as well.

[codecov-commenter]

Codecov Report

Merging #708 (17c49b2) into develop (c23318e) will not change coverage.
The diff coverage is n/a.

❗ Current head 17c49b2 differs from pull request most recent head f066f40. Consider uploading reports for the commit f066f40 to get more accurate results

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  Hits         22064    22064           
  Misses         870      870           

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