Improvements to donkey UI and donkey command

[DocGarbanzo]

Adding small improvements/fixes to the UI and training and add a new donkey command

For training:

• In the image augmentation for training, increase default MULTIPLY range from [0, 1.5] to [0, 3.0] as sunlight effects seem to have larger brightness effects than 1.5
• Set default MODEL_CATEGORICAL_THROTTLE_RANGE to 0.8, as 0.5 is just too low. We have many tubs w/ throttle values larger 0.5.

For the UI:

• Fix small issues in the UI, in particular in the car screen, like the file transfer bars and error handling
• Add option to delete pilots from the pilot database
  

For the donkey command

• Add a new command donkey models which displays the trained pilots in the shell in the same way as in the UI.
  

General:

• Bump version to 4.3.1