Allow Nightly update disabling to be targeted to specific platforms

[rvandermeulen]

Right now, if we need to turn off Nightly updates in an emergency situation, it happens across all platforms even if the problem is specific only to one. We can manually go in and add no-update rules and the like to work around that, but it would be nice if we have a bit more ability to target it like with checkboxes or something.

[bhearsum]

This is a great idea. It's not necessarily easy to implement, as platforms are just data in the Releases (and sometimes Rules), and the emergency shutoffs don't know anything about them...but no doubt we can find a way to make it work.