[web] Introduce onClearText to Search

Summary:
issue: https://linear.app/comm/issue/ENG-3466/add-some-state-for-keeping-the-search-query
In message search when a user clears the query we will need to remove the query from context. For this I need to ba able to pass a callback to the Search component. The flex: 1 was added because in
message search modal we want to have a button next to it, and have the two of them take up the full width of the modal.

Test Plan: Tested that a provided callback is called on clear

Reviewers: kamil, kuba, michal

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8273