[dagit] Do not disable Launch buttons
Instead of disabling launch-run buttons when websockets aren't connected, enable the button and show an error when the run cannot be launched -- including for situations where the webserver is down.
This allows users to launch runs even if they are unable to use websockets.
With webserver running, verify that I can launch runs from playground and via old runs.
Kill webserver with playground page open. Attempt to launch run, verify that the error dialog appears.
Repeat above on Run page. Verify that error appears.
Reviewers: alangenfeld, dgibson, bengotow, jordansanders
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D8219