Extend binary cards to work with up to 24 bits

[timbellnz]

Description

The cards activity was intended for up to about 8 bits, but it works nicely for 16:
http://csfieldguide.org.nz/en/interactives/binary-cards/index.html?digits=16

However, that makes you want 24 bits, and this seems to hang (on Chrome Version 56.0.2924.87 (64-bit) mac):
http://csfieldguide.org.nz/en/interactives/binary-cards/index.html?digits=24

It would be great if it could extend up to 24 bits or more (the images could just be grey??!), but if it's too difficult (e.g. the numbers will get very big), at least we could put in a check that the number of bits requested is within a feasible range.

Checklist

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