Don't erase old output before the new one is ready

[novoselt]

See http://ask.sagemath.org/question/33024/prevent-sagemathcell-from-clearing-output-window/

[nbruin]

Perhaps don't make it the default, though: If you leave the old output there while the new input is displayed above, the screen is in an inconsistent state. If for some reason the network is delayed a bit, you could end up with a "wrong answer" being displayed for quite a while.

[novoselt]

The spinner showing computation in progress should be much more obvious, I think - like big and in the middle of the page. Then it would be clear that what is shown is not quite what it should be.

[paulmasson]

Ignore this commit reference. Just figuring out how to get commits to show in issues in another repository.