Add arrow key controls to emoji and reaction pickers (#10637)

* Add arrow key controls to emoji and reaction pickers

* Iterate types

* Switch to using aria-activedescendant

* Add tests

* Fix tests

* Iterate

* Update test

* Tweak header keyboard navigation behaviour

* Also handle scrolling on left/right arrow keys

* Iterate

src/components/views/emojipicker/ReactionPicker.tsx

@@ -135,6 +135,7 @@ class ReactionPicker extends React.Component<IProps, IState> {
             <EmojiPicker
                 onChoose={this.onChoose}
                 isEmojiDisabled={this.isEmojiDisabled}
+                onFinished={this.props.onFinished}
                 selectedEmojis={this.state.selectedEmojis}
             />
         );