Combine Manual and Automatic Sync Preferences

Continuing the mission to rid the world of lengthy preference screens, I figured we can remove the Manual Sync preference and make it an entry in the Automatic Sync list. Now `Never` is the same as manual sync, and `When app is running` is the default setting where syncing is done automatically but not periodically in the background. The text could probably be better. Also, there are no translations for any of the sync periods selections.