Added placeholder for source plugin to read from an existing mkv file.
Added corresponding worker placeholder to transcode an mkv file using
ffmpeg.
Updated the sources page to show which sources come from which plugins.
Added link to jobs page to fix any issues caused by non-synchronised
time between webserver/worker machine leaving jobs permanently in the
running state.
Prevented the insertion of new status records unless the state is
actually being changed.
Made the HandBrake worker process write the status complete message
instead of the run-jobs dispatched, in case it crashes.
