Added Save As shortcut

[palewire]

Accel Shift S to keep it close to the existing save shortcut.

[ellisonbg]

Thanks! Two things we are wondering about:

1. What is the common shortcut for this?
2. Does this conflict with any browser shortcuts.

We are open to this, but would like to get a sense of these. Assigned to @jzf2101 to help look at these things.

[ian-r-rose]

I looked at a several applications on my system (Ubuntu) and found this to be a pretty common keyboard shortcut. On Chrome this looks like it does not preempt any browser shortcut. On Firefox it looks like this overrides a shortcut for opening the debugger, but I don't view that as a serious problem, since that is a pretty specialized use-case, and there are many ways to open it.

I think we should move forwards with this. Thanks @palewire!

[graidl]

This shortcut as well as few others that are mentioned now as new in the changelog to jupyterlab version 0.33 have been removed in thenlater commit ee815a0. I wonder why. Either this feature also needs to be removed from the changelog, or the removal needs to be reverted.