Clear logs upon soft restart and restart button

Summary:
As we discussed in HipChat, it can get confusing to figure out what logs
are from a previous run and what are from a current run. This happens a lot
with the Algorithms challenges. This makes it so that logs are cleared
upon Restart button / Program.restart and non-draw-loop injected code.
This involves live-editor and ProcessingJS change so I want to get your
opinion on the approach before I submit both changes separately.

Test Plan:
- Put a program like:
 ```

var sumNumbers = function(one, two) {
    return one + two;
};
var x = random(5);
println(sumNumbers(1, 3));
println(sumNumbers(2, 3));
println(sumNumbers(4, 5));

```
- Change the numbers, see the previous logs cleared and the new logs show up instead

Reviewers: john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D13523