Allow users to delete scheduled runs from their history

[duckduckgrayduck]

We can disable scheduled runs, but there isn't currently a way to tidy up the list, for which some of our Klaxon power users can be quite long. We should be able to do this.

[eyeseast]

Looking at the docs, I don't think there's a DELETE handler for add-on events. (Or maybe it's there but not documented.)

https://help.muckrock.com/API-19ef889269638147bbb7d8cc8af8e0fc#19ef889269638188804dc87f1677afaa