* Use `LabeledButton` instead of custom `<button>`
* Use `Icon`, `Frame` components where useful
* Reduce local/custom CSS
* Modernize tests
* Add title to `Icon` for a11y

Fixes https://github.com/hypothesis/support/issues/241

Part of https://github.com/hypothesis/client/issues/3876

Part of https://github.com/hypothesis/frontend-shared/issues/232