Simple job management page

Also added a seperate `job-indicator` for every job state
(pending/awaiting retry and failed)
The jobs page is automatically reloading when jobs are
enqueued, retried or failed.
Just keep the most recent 100 failed jobs + small improvements
Added specs for the HuginnScheduler
Just update the job list if no modal is shown
Sort queued jobs by next run time, allow to queue failed jobs
Merge pull request #468 from dsander/job-management

Simple job management page