[web] add feedback if user list is empty

Summary:
If the user has typed in something to the search box, but no results are present we should show the user some sort of feedback to make it very clear that there are no results found.

For context, here is what is shown in the figma:

Depends on D10712

Test Plan:
Please see the demo video below

Reviewers: ashoat, atul, inka

Reviewed By: ashoat, atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10714