[web] Add message search button

Summary: issue: https://linear.app/comm/issue/ENG-3461/add-magnifying-glass-button-to-chat-header

Test Plan:

(showing for the record that the pinned messages banner is not bound to message search)

Reviewers: kamil, kuba, michal

Reviewed By: michal

Subscribers: ashoat, tomek, ted

Differential Revision: https://phab.comm.dev/D8302