Add indicator that a workflow can be triggered manually in sidebar #4558

Example: https://github.com/fregante/github-url-detection/actions/workflows/npm-publish.yml

<img width="720" alt="Screen Shot 9" src="https://user-images.githubusercontent.com/1402241/125249479-7a489300-e31f-11eb-891e-bfd52c6da26b.png">

We could show a rocket or play icon in the sidebar to show that a workflow is actionable.

next-scheduled-github-action acts on exactly the same spot, so it might make sense to rename the feature and extend it, also to avoid possible race conditions on workflows that are both scheduled and actionable.