[web] Add ID selector to search bar, to avoid weird hot reload side effects

Summary:
Linear issue: https://linear.app/comm/issue/ENG-2775/white-frame-inside-of-search-bar
After some changes to a different part of the code, I was seeing a white frame inside of the search bar, due to a style from styles.css (for textarea) taking precedence over this style. This was happening most likely because of hot-reload.
It was happening when I was saving files or checking out a commit. But once I reloaded the page it was back to normal. It wasn't happening on production. So I'm adding this only so that other devs are not alarmed in the future.

Test Plan: Run web app (dev mode) on a different commit. Checked out to this commit (this used to cause the frame to appear). Checked that the frame is not present.

Reviewers: przemek, michal, bartek, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6338