Gallery of block images

[Jaqster]

This has been a request we've heard from several folks writing curriculum.
They would like access to a gallery (or maybe a generated zip file?) of all the core blocks in the Toolbox for a given editor, in a common image format (jpg, png).
This is for creating documents like this:

[pelikhan]

We can generate this in an html page on fly and people can print it to pdf.

[pelikhan]

#6776

[pelikhan]

Needs to be deployed.