[dagit] Show a tooltip on InstanceWarningIcon
In the updated left nav, the Instance warning icon could be a little more informative. This adds a tooltip with a summary of what's prompting us to show the icon at all.
This way, if a user notices it, they can quickly see what's going on and know whether or not to investigate further.
Test Plan: Load Dagit, force a repo error and keep daemons not running. Verify correct tooltip rendering. See screenshots.
Reviewers: bengotow, prha, dgibson, johann
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D6574