enhanced diff/review

cyber-dojo has two new diff/review features.

The traffic-light navigator now has buttons to move to the first (left-most button) or last (right-most button) traffic-light.

Also, if you are in a group-exercise, there is an animal-navigator to move you to different animals.

Note that the [revert] button is now called the [checkout] button. This is because you can checkout (as in git checkout) the files from any traffic-light from any animal (rather than only reverting back to one of your own previous traffic-lights). If you're doing larger exercises this can be useful if you want to 'converge' everyone to the same point.

Have fun!