Implement undo

[grossamos]

After rethinking the UX of the main logging page, I'd like to change it to look something like the mockup below.

1. We'll remove the sets dfialogue (it never really made much sense in practice)
2. Create a new undo button (basically deleting the most recent ExerciseInstance). I've messed up entering stuff in the past, so I really think it is a needed feature.