CMS: Refactor IMU menu to use pid_t instead of ad-hoc arrays

[fiam]

• Fix read past array end as noted by @flybabo on CMS: Add support for editing FF term in the PID tuning menu #4795
• Move all static variables to the top of the file
• Add macros for the PIDFF entries so the min/max/step are
  only defined in a single place