Appendix indicators should have hover help text

[benbalter]

Like when you hover over the icons elsewhere:

[mlinksva]

Pointer on where to look to begin implementation from #406 (comment)

Note that I've not made the license property tooltips you can see eg in /licenses or on individual license pages work (hover over eg "commercial use" on one of those pages). The CSS and Javascript (relevant files are assets/css/application.scss and assets/js/app.coffee) for those tooltips currently require a specific structure with bullets, but probably could be made more flexible. An alternative that might even be more useful would be a legend.