Refactor addon error modal

[Tropix126]

This PR refactors the Addon Error Modal with a few fixes.

• Use ModalHeader/ModalContent rather than DiscordClasses since they broke and were incorrectly styled.
• Use <details>/<summary> elements over custom <div>-based expanders for better a11y and simplicity.
• Make the <Divider /> component semantically accessible using the horizontal rule tag (implied ARIA role of separator).
• Add proper focus styling to expander cards to indicate keyboard navigation.
  

[Tropix126]

If/when the store is merged I can bring over a common TabBar component that's accessible as well. Store includes this now, so I can deal with the merge conflicts there.

[rauenzi]

Just remove the one file, otherwise LGTM

[rauenzi]

Much like your other PR I will probably try to squash this into the current codebase after 1.10.0