Add better support for long lists of backrefs

[agjohnson]

I'm not really sure the severity here, but display isn't great, as the backref column on multiple docutils versions can end up taking up most of the grid width.

This was brought up in #1381, there are some example there of what is happening.

A few fixes might be:

• Deep CSS hacks to convert the backref list to a dropdown or popup
• Remove all of the punctuation and let the column actually wrap
• Deep Sphinx hacks to make backref output better -- more metadata, accessibility feature, more helpful HTML syntax