Redirect to index page by status when discarding a job

This is a follow-up to #157, which redirected back after discarding
instead of redirecting to the failed job page always, since you can
discard a job from the scheduled jobs page as well. However, this
doesn't work in the case of discarding a job from its individual page,
because in that case we'd try to redirect to the job, and the job no
longer exists, which results in a 404.

With this change, we just redirect to wherever the status of the
discarded job is.

app/controllers/mission_control/jobs/discards_controller.rb

@@ -3,11 +3,16 @@ class MissionControl::Jobs::DiscardsController < MissionControl::Jobs::Applicati
 
   def create
     @job.discard
-    redirect_back fallback_location: application_jobs_url(@application, :failed), notice: "Discarded job with id #{@job.job_id}"
+    redirect_to redirect_location, notice: "Discarded job with id #{@job.job_id}"
   end
 
   private
     def jobs_relation
       ActiveJob.jobs.failed
     end
+
+    def redirect_location
+      status = @job.status.presence_in(supported_job_statuses) || :failed
+      application_jobs_url(@application, status)
+    end
 end