Fix cancelling jobs

A request to cancel a job was not processed correctly. The cancelling
routine of a task must run, regardless of the (non-final) state. Now
it works like this: if a job is currently running, it is interrupted
and its cancel routine is invoked. It then enters "cancelled" state.
If it is stuck, it is loaded and only its cancel routine is run. If it
is in a final state or waiting, it is removed from the queue.

build.sbt

@@ -307,6 +307,7 @@ val joexapi = project.in(file("modules/joexapi")).
     name := "docspell-joexapi",
     libraryDependencies ++=
       Dependencies.circe ++
+      Dependencies.http4sCirce ++
       Dependencies.http4sClient,
     openapiTargetLanguage := Language.Scala,
     openapiPackage := Pkg("docspell.joexapi.model"),