batches: add alert for active executions to batch change page
Created by: courier-new
Closes https://github.com/sourcegraph/sourcegraph/issues/32589.
https://user-images.githubusercontent.com/8942601/158302664-f8eeb3e4-d7ce-47a2-8cd6-425454614e5d.mov
Test plan
Added a new storybook story to verify the appearance of the banner with different combinations of active and complete batch specs. Manually verified the appearance and behavior of clicking "executing" on the banner from any tab of the batch change page after starting a new spec execution, as partially demonstrated in the above video.