[web] Add footers to message search modal

Summary:
issue: https://linear.app/comm/issue/ENG-4102/add-footers-to-message-search-modal
See the designs in the linked issue

The loader appears whenever no other footer is visible

Test Plan: checked that the right footers appear in the right circumstances

Reviewers: kamil, michal, kuba

Reviewed By: michal

Subscribers: ashoat, tomek, ted

Differential Revision: https://phab.comm.dev/D8241