Scratch space should highlight links

[karth295]

The most common use of the scratch space (so far) has been to paste links. We should make them <a href="..."> links so that people can click on them.

[karth295]

It might be impossible to show render links while the box is being edited. Instead, we should split into two modes: editing or not. If a box is being edited, it doesn't render links. If not, then it renders all links as <a> tags.

[karth295]

Interestingly, you can underline text in a contenteditable div, but even if you use <a href="...">, you can't click on the link: https://stackoverflow.com/questions/52029340/js-contenteditable-underline-not-working