Update manager "check again after x minutes" should be centralised

[peppy]

Currently this is implemented locally in SquirrelUpdateManager:

osu/osu.Desktop/Updater/SquirrelUpdateManager.cs

Lines 132 to 136 in 7bc8908

	if (scheduleRecheck)
	{
	// check again in 30 minutes.
	Scheduler.AddDelayed(() => Task.Run(async () => await checkForUpdateAsync().ConfigureAwait(false)), 60000 * 30);
	}

But this really should be managed at a higher level.