added: SourcesController 'new' action; path normalization for source
[scanning job] fixed bug that caused scanner jobs to be added tons of times

Fixed by adding a scanning flag to the Source model that is flagged when job is enqueued and unflagged when finished (regardless of if the scan was successful)
[directory scanner job] check if file is supported before processing
[maintenance jobs] added purge orphaned images job, added to job scheduler
[PurgeOrphanedTracksJob] removed logging