Add robotWidth and robotLength to RobotConfig

[tbeadle]

Is your feature request related to a problem? Please describe.
I would like to be able to define our robot's dimensions (including bumpers) in one place and, since it's already configured in PathPlanner, it makes sense to grab it from there like we do for other attributes of RobotConfig. This is useful for, say, setting our "zero" position that takes the size of the robot into account.

Describe the solution you'd like
Add robotWidth and robotLength attribute to RobotConfig. These seem to only be accessible in the UI components for rendering the robot image currently.

[tbeadle]

It would also be useful to have the gear reduction in there.