Changes

** Problem*** Currently sync uses the 'clients' sync data collection as a list of active devices. This list has no state and difficult to 'delete' an device off list. Most client display a device if it's been active within the last two weeks. This design creates a number of bugs visible to the user.** Plan*** All devices, Desktop, Android, iOS register with FxA*** There should be events that removed devices from this list, such as uninstall, reinstall, or devices with the same name conflicts.*** FxA device manager endpoint provides real-time list of active devices to sync clients to display.** [https://accounts.firefox.com/signin?forceDeviceList=1 FxA device list manager *WIP*] ** Current Status*** Pending on device registration on iOS and fixing issues*** Pending fixes to settings display*** Pending work to improve the accuracy of the device list controlled by user events.