[web][native] Use userStoreMentionSearchIndex

Summary:
This diff replaces old userStoreSearchIndex usage in chat-input-bar.react.js with new userStoreMentionSearchIndex selector.

Depends on D8901.

Test Plan: Tested later in the stack.

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8902