Manage Shuffleboard pointers in individual software subsystems

[jkleiber]

Summary
When the WPILib updated Shuffleboard to return pointers in GetEntry(), our anti-pattern of managing global references no longer made sense. In #26 , we ported the references to pointers to maintain compatibility, but it would be better to track our shuffleboard widgets in the software subsystems instead of globally.

Work to Do

• Move shuffleboard pointers to the respective software subsystems

Verification

• CI Passes
• Shuffleboard entries moved correctly