[dagit] Add assets to global search
Add assets to global search, using a secondary query separate from the bootstrap query. This will allow us to show bootstrapped results first, just in case the asset query is slow.
Also added a spinner to show when a query is underway.
Test Plan: Load dagit, open global search. Search for an asset name, verify that results appear, with bootstrapped objects first in the list.
Reviewers: prha, sandyryza
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D7218