Improved mobile device detection support.

- Mobile device detection tries to read from an X-UA-Device header to take direction from a web server.
- Added support for distinguishing between desktop, mobile, tablet, and app.
- Adding "VanillaMobileApp" to a user agent now makes Vanilla think it's a mobile app.
- Changed the mobile/desktop override cookie to use X-UA-Device-Force which is no longer a boolean.
Using two variables of the same name with difference casing is the path to the dark side. `$Unset` gets used again below.