Backport PR #5459: Fix interact animation page jump FF

Firefox doesn't render images immediately as the data is available.  When animating the way that we animate, this causes the output area to collapse quickly before returning to its original size.  When the output area collapses, FireFox scrolls upwards in attempt to compensate for the lost vertical content (so it looks like you are on the same spot in the page, with respect to the contents below the image's prior location).  The solution is to resize the image output after the `img onload` event has fired.

This PR:
- Releases the `clear_output` height lock after the image has been loaded (instead of immediately or using a timeout).
- Removes a `setTimeout` call in the `append_output` method.
- `clear_output` in zmqshell no longer sends `\r` to the stream outputs.

closes #5128
Backport PR #5488: Added missing require and jquery from cdn.

For some reason (I suppose some changes at the css level) the font size inside the input cells was fixed at 14 px... making the fonts really small in the reveal slideshows. This is really annoying...
As a plus, I have also added the missing calls for require and jquery (as the full html template does).
I think these fixes belong to 2.0, but I also know we are on the edge... so I hope to get it inside :wink:

Cheers.
Backport PR #5498: use milestones to indicate backport

instead of labels.

I think this will more clearly indicate what's new in what releases, plus it's way easier to show pull requests with a specific milestone than it is with a specific label.
Backport PR #5512: Update tooltips to refer to shift-tab

Closes gh-5510
Backport PR #5522: fix iteration over Client

I didn't intend for Client to be iterable, but defining `__getitem__` makes it iterable as long as engine IDs are continuous (not always the case).

This explicitly defines `__iter__` so it behaves consistently.

closes #5519
Backport PR #5524: never use `for (var i in array)`

because javascript is terrible, and allows code to make this unsafe and behave in arbitrarily wrong ways.
Backport PR #5535: fix n^2 performance issue in coalesce_streams preprocessor

for n consecutive stream outputs, `\r` fix would be compiled n times, and applied to each ith output (n-i) times.

- move pattern to module level
- apply replacement after coalescing outputs

An example notebook from nbviewer with ~1k outputs that was taking 90 seconds to render now takes 3 seconds.
Backport PR #5559: Minor typo fix in "Cython Magics.ipynb"

``IPtyhon`` -> ``IPython``
Backport PR #5507: Fix typo in interactive widgets examples index notebook

This is a really trivial fix. Haven't read about the signatures / new security features -- let me know if I need to do something there...