Improve accessibility (part 4) (#4408)

* fix(dropdown_menu): Keyboard navigation

* fix(icon_button): Add aria-pressed attribute

* fix(privacy_dropdown): Make accessible

* fix(emoji_picker_dropdown): Make accessible

* fix(icon_button): Support tabIndex

* fix(actions_modal): Remove icon from tab order

* fix(dropdown_menu): Add role=group

* fix(setting_toggle): Toggle via space key

* fix(dropdown_menu): Remove redundant handling of Space key

* fix(emoji_picker_dropdown): Remove redundant Space key handling

* fix(privacy_dropdown): Remove redundant Space key handling

* fix(status): Switch to article and add aria-posinset, aria-setsize

* fix(status_list): Use role=feed and pass more ARIA props to Status

* chore(eslint): jsx-a11y/role-supports-aria-props