[lib] Memoize userinfos in usePotentialMemberItems

Summary: This addresses the urgent issue outlined in [[ https://linear.app/comm/issue/ENG-6139/trying-to-add-members-to-a-chat-freezes-the-web-app | ENG-6139 ]]. I should've memoized the `userInfos` object.

Test Plan: Since `usePotentialMemberItems` is used in both `chat-thread-composer.react.js` and `add-members-modal.react.js` (both also mentioned in the original bug report), I made sure that those both no longer completely freeze/crash.

Reviewers: ashoat, michal

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10409

lib/shared/search-utils.js

@@ -104,7 +104,8 @@ function usePotentialMemberItems({
   +inputCommunityThreadInfo?: ?ThreadInfo,
   +threadType?: ?ThreadType,
 }): UserListItem[] {
-  const searchIndex: SearchIndex = useUserSearchIndex(values(userInfos));
+  const memoizedUserInfos = React.useMemo(() => values(userInfos), [userInfos]);
+  const searchIndex: SearchIndex = useUserSearchIndex(memoizedUserInfos);
 
   const communityThreadInfo = React.useMemo(
     () =>