Auto download of updates should be configurable

[mscharley]

Is your feature request related to a problem? Please describe.

Whether or not to automatically download updates should be configurable, both at a personal level and at a packaging level.

1. At a personal level, automatic updates are not always the best option for eg. laptops that are frequently on tethered mobile plans
2. At a packaging level, if this is packaged as eg. a linux package, then the package manager won't take nicely to the application auto-updating itself over the top of the managed installation.

Describe the solution you'd like

1. A configuration option in the config file, exposed in the configuration panel.
2. Until I hear from a package maintainer, I'm just going to expose an environment variable that can be used to forcefully disable updates and hide any UI related to them.