types: Make a few Props definitions explicitly inexact.

An instance of zulip#3452.

PR zulip#4465 is currently open for type-checking `MessageList`'s props.

src/message/messageUpdates.js

@@ -2,9 +2,12 @@
 import isEqual from 'lodash.isequal';
 import type { Narrow } from '../types';
 
+// TODO: Consider completing this and making it exact, once
+// `MessageList`'s props are type-checked.
 type Props = $ReadOnly<{
   narrow: Narrow,
   messages: $ReadOnlyArray<$ReadOnly<{ id: number }>>,
+  ...
 }>;
 
 type TransitionProps = {|

src/webview/handleOutboundEvents.js

@@ -146,13 +146,16 @@ export type WebViewOutboundEvent =
   | WebViewOutboundEventMention
   | WebViewOutboundEventTimeDetails;
 
+// TODO: Consider completing this and making it exact, once
+// `MessageList`'s props are type-checked.
 type Props = $ReadOnly<{
   backgroundData: BackgroundData,
   dispatch: Dispatch,
   messages: $ReadOnlyArray<Message | Outbox>,
   narrow: Narrow,
   showActionSheetWithOptions: ShowActionSheetWithOptions,
   startEditMessage: (editMessage: EditMessage) => void,
+  ...
 }>;
 
 const fetchMore = (props: Props, event: WebViewOutboundEventScroll) => {