public worksheets on *.sagenb.org

[Jason Grout]

I've implemented some sanitizing of public worksheets [1] and applied it
to demo.sagenb.org as a test. The concerns from before were that
javascript was executing on the page, leading to malware being on the page.

Can people test the new html sanitizing being done on demo.sagenb.org?

If it looks good, I'll roll it out to the other *.sagenb.org servers and
we'll turn published worksheets back on.

Thanks,

Jason

[1] https://github.com/sagemath/sagenb/pull/100