Add a USB device blocking popup.

[tnek]

• Refactor SNTNotificationManager and SNTMessageWindowController for generalized notification logic
• Add the following popup:

[russellhancox]

I haven't had a chance to look at the code yet but from the screenshot I have a few immediate comments:

1. Can the message shown be customized or can a custom message be added on the window? This can be useful for admins to point users at company specific documentation for the messages.
2. Can the remount permissions have "human readable" conversions added?
3. Can we remove the large amount of empty space between the event details and the OK button?
4. Can the OK button be centered horizontally?

[tnek]

Done. They'll be configurable with the BannedUSBBlockMessage and RemountUSBBlockMessage configurator keys. Here's what these prompts look like now:

I'm struggling with phrasing for human-readable conversions of the remount permissions, if you have ideas for what's clearest for users.