[web] Introduce search modal component

Summary:
We're going to have a couple of modals that have search functionality, so it makes sense to have a component for that.
We usually prefer using children components when introducing containers like this, but in this case that approach has a couple of disadvantages: we would need to pass searchText to children as a prop and that would require using React.cloneElement. Also, defining child component props would be complicated to allow supporting all the components that have searchText props. None of these are too serious, but it looks like using children function is more readable and easier approach. If you disagree, it can easily be modified.
Depends on D3385

Test Plan: Open members modal and check if filtering works correctly.

Reviewers: benschac, atul, def-au1t

Reviewed By: benschac, atul, def-au1t

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3494