Dfn panel changes for linking syntax

[dlaliberte]

This PR adds a "Possible linking syntaxe(es)" section to dfn panels.

• The copy icon is perhaps overkill.
• The cursor for the dfn is changed to be a 'help' cursor, since the dfn is not a link to anything but the dfn panel.
• There are also a couple css changes regarding dfn panels in general, to maybe fix some whitespace issues, depending on what was intended.
  • Avoid wrapping after section numbers.
  • No ellipsis needed most of the time.
  • Multiple reference numbers no longer wrap (e.g. (2) (3) (4))

Here is what it looks like now.

[dlaliberte]

Without event.preventDefault(), the page scrolls so that the dfn is at the top of the window, and the dfn panel appears to just disappear, but it is moved simultaneously to the bottom left corner. If that is your intention, fine. I found it confusing, however. I would guess you intend this behavior to make it easy to revisit the dfn by clicking the header link in the previously activated dfn panel. I so, maybe we should think about doing that and more in a better way.

[dlaliberte]

I hadn't realized that this event.preventDefault() call was also disabling all the other links on the panel. We'll look into fixing the scrolling behavior perhaps a different way.

[dlaliberte]

This PR fixes #1319