A working CHK download demo for the TechDemo page

This integrates tahoe-chk, tahoe-great-black-swamp, and gbs-downloader to add a TechDemo widget where a CHK capability string can be entered and the UTF-8-encoded plaintext it is associated with will be rendered into the page.

It comes with a hard-coded grid configuration that only works on my home LAN. I filed #59 (closed) for improving that situation.

This should largely complete %Mobile: Arbitrary N/k CHK .