[web] Set emoji Picker to "dark" mode and set search box to autoFocus

Summary:
Current emoji Picker for reactions changes theme based on OS mode.

Instead we set to always be dark to match rest of web.

We also set autoFocus to true so user can begin typing as soon as the emoji picker is opened

Test Plan: Looks as expected

Reviewers: ashoat, ginsu

Reviewed By: ashoat, ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8362