Install-time option to disable updates over the network

[philrz]

One of the findings that came from #2685 was that there may be conservative environments (a government agency in this case) where Zui's ability to update itself over the network is undesirable. Quoting the user:

...we cannot allow automatic download and install regardless of whether we are running in an Admin or standard user context. How can this be disabled?

In that case, just having the new Check for Updates settings added via #1211 would probably be inadequate since a user could conceivably just go into Settings and re-enable auto-updates or select the Check For Updates option in the pull-down menu. Therefore to meet this requirement I expect we'd need to provide an install-time option so that the app would prevent updates over the network under all circumstances and not allow the user to change this.

Note that #2685 was written in the context of a managed Windows environment, and if we were to add this functionality it might be something we'd only do on a specific platform like Windows.