Make shuffleboard entries global pointers in port

[jkleiber]

Summary
Using global references (using extern) for the shuffleboard stuff was working great in beta3. Now in beta7, they've switched to using pointers for some reason, and we need to change to pointers in order for the code to compile.

The argument can be made that we should not be doing what we are currently doing (storing references to shuffleboard variables globally), but this pattern is driven by the WPILib's setup. We can reconsider our decisions in another ticket, and fix the code quickly in this one so we can build code on beta7

Work to Do

• Switch global references to global pointers

Verification

• CI passes

Deferred work
-#28: Maintain the shuffleboard pointers in software subsystems

FYI @danielbrownmsm