Disable file syncing controls absent sync server settings

[ahoward]

In order to make it more clear that file syncing cannot function independently of library syncing, disable the file syncing controls in the sync preference pane when there are no Zotero sync server settings present. Just make sure the existing file syncing preferences are still saved. I don't think it matters if people can't change them. They can always put in junk data for the sync server if they care that much.