Fix aborting work request from "Waiting for signature" card

We can't use a plain link for this, since it needs to be a POST request. Use a button styled as a link instead; I think this is OK since it's clear enough from the text that it's going to perform an action.

I didn't add a news fragment for this, since it's a correction to the fix for #384 (closed) which hasn't been part of a release yet.

Fixes: #880 (closed)