CC-5709: Airtime Analyzer

* Overhauled Add Media screen, now shows state of recent uploads
* Dropped old unused "state" column, added new file_import column to cc_files
* New PluploadController methods
* Save the filename as the track title for unprocessed uploads
* Hide pending files from the library until they've been processed.
* Don't overwrite files with duplicate names, we rename them instead.
Merge branch 'cc-5709-airtime-analyzer' of github.com:sourcefabric/Airtime into cc-5709-airtime-analyzer