Add option to disable app auto-update

[mmmarcos]

Alternatives suggested by @Max-7:

1. Disable it directly on the Electron side, and simply do not instantiate the updater.
   • The easiest way to do this at present is in hard code, but can also be done via the config file with a bit of fiddling, not very complicated.
2. Disable the event on the app frontend so nothing will be displayed.
   • This is the case for web applications, for example, where electron doesn't emit an event to tell you that an update is available.

The first is probably the best, as there's no point in requesting github and downloading if you want to disable it. It's not complicated to do, but it's a pain to test.

[mmmarcos]

Related issue: #7100

[mmmarcos]

Discussed today: use an ENV var to create a specific build that does not check for update.

The option for the end-user (issue #7100) can be implemented later.