Glossary quality check script

One thing the QA checklist 
<https://www.w3.org/WAI/GL/wiki/WCAG_2.1_QA_Checklist> mentions is that 
the definitions of terms in WCAG 2 should be able to be replaced 
directly into the SC and things still make sense. To test this, I added 
a script that will substitute definitions in place of terms, if the 
"defs" parameter is present in the URI, i.e.,

https://rawgit.com/w3c/wcag21/master/guidelines/index.html?defs

The script only substitutes the first line of the definition, since that 
is the part that's meant to be drop-in replaceable. It puts the original 
term that was linked into the title attribute of the replaced link, to 
help figure out what was originally there (I could also make that more 
directly visible if people want). The script may only work with newer 
browsers, and only if the base script has run - there should be a 
"Respec" button in the top right fairly early in the navigation order, 
and the page may need to be refreshed if that hasn't shown up.

A few observations / comments:

  * Some SC become very unwieldy when expanded this way, though that's
    why we moved a lot of content to terms;
  * There are WCAG 2.0 terms that don't work out well this way, but I
    don't think we will fix that, this is mainly to look at WCAG 2.1
    quality;
  * The group may decide it wants to be less particular than me about
    exact drop-in ability of definitions, but I still think it's a
    useful exercise and a rough goal we should maintain for consistency
    and understandability;
  * Some SC may become fairly nonsensical and that should be a cue that
    our definition isn't working well and needs improvement.

I'll leave it at that for now, people can look over the guidelines with 
this filter as it suits them and raise any flags you see. I expect 
eventually to file issues on terms I think really aren't working out 
seen in this context.

Michael