Speed up switching between drive modes

Currently, there is a lot of reconfiguration that goes on when switching between drive modes. This can result in delays to switch modes.

See [drivetrain-config](/FRC2706/2019-2706-Robot-Code/tree/drivetrain-config)

- [X] Use PID slots instead of sending new PID values
- [X] Try to reconfigure sensors as little as possible
- [X] Use different Talons for different sensor configurations
- Front left/right for individual control
- Back left combined drive forward
- Back right combined rotation
- [x] Use `SlotConfiguration` class to configure slots rather than setting speed manually

Start by reviewing the drivetrain-config branch and the drive-mode switching implementation it contains. Compare the current switching behavior with the listed completed changes, then verify that switching modes no longer incurs unnecessary reconfiguration delays.

Written by the indexing model from the issue text.