On-demand download support

Can choose to download files "at sync time" or "as needed"

On-demand defaults to on, but remains off for existing users

To-do:

- Handling of local and remote file changes on on-demand download
  (currently if a file exists it isn't downloaded, which means a
  remotely modified file won't be redownloaded in on-demand mode)
- Additional control over file downloading and retention

Other changes:

- Overhauled entire file syncing architecture
- Replaced numAttachments column with Note and Attachment columns with
  dynamic icons to indicate status
- Double-clicking a parent with a missing best attachment and on-demand
  downloading off no longer loads the parent URL
- Bugs

components/zotero-service.js

@@ -96,7 +96,12 @@ const xpcomFilesLocal = [
 	'style',
 	'sync',
 	'storage',
-	'storage/session',
+	'storage/streamListener',
+	'storage/eventManager',
+	'storage/queueManager',
+	'storage/queue',
+	'storage/request',
+	'storage/module',
 	'storage/zfs',
 	'storage/webdav',
 	'timeline',