fix: message input not preventing default on enter on React 16 (#2380)

React 16 had a wacky thing called [event
pooling](https://legacy.reactjs.org/docs/legacy-event-pooling.html).
Basically you couldn't use event objects outside of the microtask in
which the event handler was triggered, unless you explicitly called
`event.persist()`.

When we made key down handler on message input async in
#2332, we broke this
rule. So `preventDefault()` in [submit
handler](https://github.com/GetStream/stream-chat-react/blob/master/src/components/MessageInput/hooks/useSubmitHandler.ts#L119)
became a no-op.

React still provides the `perist()` method on synthetic events, even in
newer versions, so we just use it to fix the issue.

src/components/AutoCompleteTextarea/Textarea.jsx

@@ -119,6 +119,8 @@ export class ReactTextareaAutocomplete extends React.Component {
     const trigger = this.state.currentTrigger;
 
     if (!trigger || !this.state.data) {
+      // https://legacy.reactjs.org/docs/legacy-event-pooling.html
+      event.persist();
       // trigger a submit
       await this._replaceWord();
       if (this.textareaRef) {