Update default code-cell to enable collapsibility

[mmcky]

It is sometimes convenient when reading a lecture to collapse a code-block (once the code has been read).

We should update the default code-cell to include an arrow that enables the collapse of the code-cell (by default).

[mmcky]

@AakashGfude do you have time to look at this?

[mmcky]

@jstac comment

I was thinking that every cell would look like this (after "Hide code cell source" is changed to "Hide code" or just "Hide")

We could even drop "Hide code" everywhere and just have the arrow, which points to the right when the cell is closed and down when it's open.

I feel like that's a convention these days....?

image.png

[jstac]