This PR implements three small measures aimed at encouraging users to keep the application running in the background for the purposes of increasing the likelihood of successful lease-renewal. These consist of:

• If the user is connected to at least one ZKAPAuthorizer-enabled storage grid, upon exiting the application, display a warning (informing users that the application will stop renewing data if it is not running -- and that doing so over extended periods can result in data-loss) and require user-confirmation in order to proceed.
• If the [default] autostart value is set to true in config.txt, enable autostart by default, launching the application automatically upon login.
• if the [default] autostart value is set to true in config.txt, display a warning and require user-confirmation in order to disable autostart.

Note that the above-described option to enable autostart by default is primarily intended for customized builds of Gridsync (such as PrivateStorageDesktop) that might wish to deploy stricter or different default behaviors; Gridsync itself will continue to ship with autostart disabled by default.