[Extractor] Replace module which with twisted.python.procutils.which
[Execute] Tabs to spaces
[#1126] [#2322] Emit FinishedEvent after moving storage complete

Also changed the Execute and Extractor plugins to process the updated
FinishedEvent functionality.
[#1290] [Execute] Add TorrentRemoved event
[Extractor] Tidy plugin code and add webui page
[#2238] [Scheduler] Fix undefined this.scheduleCells